structure du paysage et Écologie comportementale …€¦ · jo gibson (cégep de la pocatière),...

YVES TURCOTTE

STRUCTURE DU PAYSAGE ET ÉCOLOGIE COMPORTEMENTALE DES OISEAUX FORESTIERS EN HIVER

Thèse présentée à la Faculté des études supérieures de l’Université Laval

dans le cadre du programme de doctorat en sciences forestières pour l’obtention du grade de Philosophiae Doctor (Ph. D.)

DÉPARTEMENT DES SCIENCES DU BOIS ET DE LA FORÊT FACULTÉ DE FORESTERIE ET DE GÉOMATIQUE

UNIVERSITÉ LAVAL QUÉBEC

OCTOBRE 2005 © Yves Turcotte, 2005

Résumé Les effets de la déforestation sur les effectifs des populations d’oiseaux ont fait l’objet de

nombreuses études au cours des dernières décennies. Cependant, relativement peu d’entre

elles ont été réalisées en dehors de la période de reproduction. Mon projet de recherche

avait pour objectifs de décrire les effets de la déforestation sur les populations d’oiseaux

forestiers dans un contexte hivernal et de mettre en évidence, à l’aide d’une approche

expérimentale, d’éventuels mécanismes comportementaux pouvant affecter la dynamique

des populations étudiées et leur répartition spatiale. Les travaux d’échantillonnage sur le

terrain ont eu lieu pendant trois hivers au Kamouraska. La structure de 24 paysages (rayon

de 500 m) a été décrite à partir d’une image satellite. Ces paysages représentaient un

gradient complet de déforestation (8 à 88% de couvert forestier). Dans la moitié de ces

paysages, de la nourriture a été fournie ad libitum pendant tout l’hiver. L’intégrité des

peuplements forestiers (une composante principale incorporant les variables couvert

forestier et densité des bordures) était positivement associée à l’abondance des Mésanges à

tête noire et à la richesse spécifique pendant l’expérience d’approvisionnement en

nourriture mais, seulement dans les paysages expérimentaux. Dans les paysages témoins,

l’abondance des Mésanges à tête noire et la richesse spécifique ont au contraire légèrement

diminué avec une augmentation de la valeur de l’intégrité des peuplements forestiers. Ces

résultats suggèrent que les paysages témoins où la déforestation n’était pas marquée ont pu

faciliter l’émigration d’oiseaux lorsque les conditions environnementales (froid, rendement

énergétique lors de la quête alimentaire) se sont détériorées. En contrepartie, dans les

paysages témoins où la déforestation était sévère, des oiseaux ont pu se trouver piégés.

Néanmoins, la structure des paysages n’avait aucun effet sur la condition énergétique des

mésanges. Seul l’approvisionnement en nourriture a eu un effet positif sur la condition des

mésanges mais aussi, sur leur patron quotidien d’engraissement. Finalement, les mésanges

des paysages témoins les plus sévèrement déboisés avaient une plus grande propension à

s’exposer en milieu ouvert à d’éventuels prédateurs, tandis que celles bénéficiant de

l’approvisionnement en nourriture demeuraient toujours en retrait, à proximité du couvert

forestier.

Abstract Over the last few decades, many researchers have addressed the impacts of forest loss on

forest bird abundance. However, most of these works were conducted during the breeding

season. The aim of my research was to document the effects of deforestation on bird

populations during winter. Using an experimental approach, I also wanted to assess the

effect of behavioral mechanisms potentially affecting population dynamics and spatial

distribution of forest birds. Field work was conducted during three winters in Kamouraska

County. The structure of 24 landscapes (500-m radius) was described from a satellite

image. These landscapes represented a broad gradient of deforestation (forest cover 8–88

%). In half of these landscapes, we provided an unlimited source of food. I evaluated the

effects of landscape structure 1) on the spatial distribution of the forest bird community, 2)

on the fattening strategies, and 3) the anti-predator behavior of the Black-capped Chickadee

(Poecile atricapillus). Forest integrity (a composite of forest cover and edge density) was

positively associated with chickadee abundance and species richness in landscapes that

were supplemented. However, in control landscapes, chickadee abundance and species

richness tended to decrease with an increase in forest integrity. This suggests that the more

forested control landscapes facilitated winter emigration when conditions deteriorated.

Conversely in highly deforested and fragmented control landscapes, birds became “gap-

locked”. Landscape structure did not affect chickadees’ energetic condition. However,

food-supplementation improved it and affected the pattern of daily fattening as well. In the

more deforested control landscapes, chickadees showed more willingness and ventured

farther into the open despite a likely increase in the risk of predation. However, where ad

libitum food was available prior to the experiment, chickadees always remained close to the

forest edge, regardless of the level of deforestation.

Avant-Propos Afin de pouvoir mener à terme ce projet de recherche, j’ai pu compter sur la collaboration

d’un grand nombre de personnes. Je suis tout d’abord extrêmement reconnaissant envers

Christine Pomerleau, Maude Pelletier, Valérie Godbout, Caroline Fournier, Florence Portal

et Lluis Brotons pour toute l’aide apportée lors de la réalisation des travaux sur le terrain.

Mille excuses pour vos engelures! L’expertise de Bruno Drolet lors de l’analyse de l’image

satellite de l’aire d’étude et de la réalisation des cartes s’est avérée des plus utiles. Bruno,

t’es un Michel-Ange de la géomatique!

Les commentaires généreux et constructifs de Yves Aubry (Université Laval), Marc Bélisle

(Université de Sherbrooke), Julie Bourque (Université Laval), Charles R. Brown

(University of Tulsa, États-Unis), Marcel Darveau (Canards Illimités Canada), Pehr H.

Enckell (Lund University, Suède), Jean Ferron (Université du Québec à Rimouski), Mary-

Jo Gibson (Cégep de La Pocatière), Caroline Girard (Université de Sherbrooke), Yrjö Haila

(University of Tempere, Finlande), Shelley A. Hinsley (Institute of Terrestial Ecology,

Royaume-Uni), Marc J. Mazerolle (USGS Patuxent Wildlife Research Center, Etats-Unis),

Jeremy McNeil (Université Laval), Ghislain Rompré (Université Laval), Jean-Pierre L.

Savard (Service canadien de la faune), Véronique St-Louis (University of Wisconsin-

Madison, États-Unis) ont, à des degrés divers, contribué à améliorer la qualité de cette

thèse. Il me fait plaisir de souligner ici l’importante contribution à cet égard de Joël Bêty

(Université du Québec à Rimouski), Thomas C. Grubb, Jr. (Ohio State University, États-

Unis), et Eliot McIntire (University of Montana, Etats-Unis), qui ont agi à titre

d’examinateurs de la thèse. J’ai aussi pu profiter de l’expertise de plusieurs autres collègues

étudiants-chercheurs, trop nombreux pour être nommés ici. Les discussions formelles ou

impromptues que nous avons eues ont indiscutablement contribué à l’évolution de ma

pensée. À vous tous, merci pour vos lumières!

J’ai également pu bénéficier du support constant de mon directeur, André Desrochers, et ce,

à toutes les étapes ayant mené à la réalisation de cette thèse. De l’élaboration du projet

jusqu’à la rédaction des manuscrits, ses conseils ont toujours été justes et des plus

pertinents. Merci André de m’avoir accordé le privilège de faire parti de ton équipe!

iv Cette étude a bénéficiée du soutien financier du Conseil de la Recherche en Sciences

Naturelles et en Génie du Canada, sans lequel, elle n’aurait pu être possible. De plus, j’ai

pu compter sur l’aide accordée par mon employeur, le Cégep de La Pocatière, par le biais

du programme de perfectionnement du personnel enseignant.

Table des matières Résumé................................................................................................................................... ii Abstract.................................................................................................................................. ii Avant-Propos ........................................................................................................................ iii Table des matières ..................................................................................................................v Liste des tableaux................................................................................................................. vii Liste des figures .....................................................................................................................ix Introduction.............................................................................................................................1

Déforestation et déclin des populations d’oiseaux .............................................................1 Causes présumées du déclin des populations .....................................................................1 Le contexte hivernal............................................................................................................2 Structure de la thèse............................................................................................................4

Chapitre I: Landscape-dependent distribution of northern forest birds in winter.............7

Avertissement .....................................................................................................................7 Résumé................................................................................................................................8 Abstract...............................................................................................................................9 Introduction.......................................................................................................................10 Methods ............................................................................................................................12

Study area and experimental design .............................................................................12 Landscape characterization...........................................................................................14 Bird surveys ..................................................................................................................15 Statistical methods ........................................................................................................16

Results...............................................................................................................................17 Landscape structure ......................................................................................................17 Principal component analysis .......................................................................................18 Landscape structure and use by birds ...........................................................................19

Discussion.........................................................................................................................31 Chapitre II: Landscape structure, food abundance and winter fattening strategies of black-capped chickadees.......................................................................................................34

Avertissement ...................................................................................................................34 Résumé..............................................................................................................................35 Abstract.............................................................................................................................36 Introduction.......................................................................................................................37

vi

Materials and methods ......................................................................................................40 Study area and experimental design .............................................................................40 Landscape characterization...........................................................................................41 Trapping and measurements .........................................................................................42 Evaluation of energetic condition .................................................................................43 Statistical analysis.........................................................................................................45

Results...............................................................................................................................47 Energetic condition .......................................................................................................47 Patterns of daily fattening .............................................................................................52

Discussion.........................................................................................................................55 The landscape context...................................................................................................55 The pre-treatment period .............................................................................................55 The treatment period....................................................................................................56

Chapitre III: Landscape-dependent response to predation risk by forest birds .................59

Avertissement ...................................................................................................................59 Résumé..............................................................................................................................60 Abstract.............................................................................................................................61 Introduction.......................................................................................................................62 Methods ............................................................................................................................64 Results...............................................................................................................................67 Discussion.........................................................................................................................69

Conclusion générale..............................................................................................................70

Applications écologiques..................................................................................................73 Perspectives de recherche .................................................................................................74

Bibliographie ........................................................................................................................78 Annexe A: Playbacks of mobbing calls of Black-capped Chickadees help estimate the abundance of forest birds in winter.......................................................................................88

Avertissement ...................................................................................................................88 Résumé..............................................................................................................................89 Abstract.............................................................................................................................90 Introduction.......................................................................................................................91 Study area and methods ....................................................................................................92 Results...............................................................................................................................94

Comparison of pre-playback and playback counts .......................................................94 Effect of time of day .....................................................................................................96

Discussion.........................................................................................................................97 Literature cited..................................................................................................................98

Liste des tableaux CHAPITRE I

Table 1. Landscape metric factor loadings and variance explained by the two principal components for the 24 landscapes. ...............................................................................18

Table 2. Number of landscapes where each species was detected, depending on the type of

landscape.......................................................................................................................20 Table 3. Mean number of black-capped chickadees detected during playback counts and

total number captured by age........................................................................................21 Table 4. Comparison of Poisson regression models for the association between landscape

structure, food treatment and the number of black-capped chickadees detected during playback counts.............................................................................................................23

Table 5. Comparison of Poisson regression models for the association between landscape

structure, food treatment and species richness during playback counts.. .....................25 Table 6. Association between landscape structure, food treatment and the number of black-

capped chickadees detected during playback counts.. ..................................................27 Table 7. Association between landscape structure, food treatment and species richness

during playback counts.. ...............................................................................................28

CHAPITRE II

Table 1. Comparison of normal regression models for the association between landscape structure, food treatment, time elapsed since sunrise and body mass index or log body mass index of black-capped chickadees.. .....................................................................48

Table 2. Comparison of logistic regression models for the association between landscape

structure, food treatment, time elapsed since sunrise and fat score of black-capped chickadees.....................................................................................................................49

Table 3. Association between landscape structure, food treatment, time elapsed since

sunrise and body mass index or log body mass index of black-capped chickadees.. ...50 Table 4. Association between landscape structure, food treatment, time elapsed since

sunrise and fat score of black-capped chickadees. .......................................................51

viii CHAPITRE III

Table 1. Description of the 12 food-supplemented and the 12 control landscapes used in behavioural trials...........................................................................................................64

Table 2. Influence of food treatment and landscape structure on the maximum distance

ventured into the open by foraging Black-capped Chickadees in winter. ....................68 ANNEXE A

Table 1. Mean species richness and number of individuals of the most commonly detected species during paired pre-playback and playback counts within an agricultural landscape in Quebec during three consecutive winters.. ..............................................94

Table 2. Number of census sites where each species was detected depending on the type of

count..............................................................................................................................95 Table 3. Mean species richness and total number of individuals during paired pre- playback

and playback counts at different times of day within an agricultural landscape in Quebec during three consecutive winters.. ...................................................................96

Liste des figures CHAPITRE I

Figure 1. The study area in Kamouraska County, Quebec, Canada.. ...................................13 Figure 2. Relationship between forest cover and Ln edge density within a 500-m radius for

the 12 supplemented and the 12 control landscapes. ....................................................17 Figure 3. Relationship between forest integrity and number of black-capped chickadees

detected during playback counts...................................................................................29 Figure 4. Relationship between forest integrity and species richness during playback

counts.. ..........................................................................................................................30

CHAPITRE II

Figure 1. Relationship between mean tarsus length and longest wing length of 212 black-capped chickadees of known age..................................................................................44

Figure 2. Relationship between time elapsed since sunrise and body mass index or fat score

of black-capped chickadees during pre-treatment and treatment periods for the two winters of the study.......................................................................................................54

CHAPITRE III

Figure 1. Three control landscapes used in behavioural trials..............................................65 Figure 2. Relation between forest cover and maximum distance ventured into the open by

foraging Black-capped Chickadees in control and food-supplemented landscapes. ....67

Introduction

Déforestation et déclin des populations d’oiseaux Les effets de la perte des habitats forestiers sur la faune et ce, tant chez les invertébrés que

chez les vertébrés, ont fait l’objet d’un nombre considérable d’études au cours des quelque

25 dernières années (Hanski 1994, Pimm 1998, Kolozsvary et Swihart 1999). Les études

portant plus spécifiquement sur les impacts perceptibles au niveau des effectifs des

populations d’oiseaux ont été particulièrement nombreuses mais aussi, parmi les plus

médiatisées (Terborgh 1989). Ainsi, de nombreux chercheurs ont-ils démontré les effets

négatifs de la dégradation des habitats forestiers sur l’abondance (Lee et al. 2002,

Schmiegelow et Mönkkönen 2002), la richesse spécifique (Freemark et Merriam 1986,

Villard et al. 1999, Boulinier et al. 2001) et la persistance des populations au fil des ans

(Aberg et al. 2000, Hames et al. 2001).

Causes présumées du déclin des populations Au cours des dernières décennies, des tendances à la baisse ont été constatées dans les

effectifs de certaines populations d’oiseaux forestiers telles que, la Grive des bois

(Hylocichla mustelina), le Pioui de l’Est (Contopus virens) ou encore, la Paruline à poitrine

baie (Dendroica castanea) (Robbins et al. 1989, Downes et Collins 2003). Le débat portant

sur la nature exacte des causes à l’origine de tels déclins se poursuit toujours (Fahrig 1997,

Harrison et Bruna 1999, Flather et Bevers 2002, Haila 2002). Néanmoins à ce jour,

plusieurs mécanismes au-delà de la simple perte d’habitat, ont pu être mis en évidence afin

d’expliquer les effets négatifs de la déforestation sur l’abondance des oiseaux forestiers et

leur répartition spatiale. La majorité des mécanismes identifiés ont cours à l’intérieur des

habitats boisés eux-mêmes et ce, particulièrement à proximité des bordures. Ainsi il a été

démontré qu’à proximité des bordures, en raison de l’humidité moins élevée du sol, certains

groupes d’insectes forestiers dont s’alimentent les oiseaux peuvent être moins abondants

(Burke et Nol 1998, Zanette et al. 2000, Van Wilgenburg et al. 2001). L’incidence de la

prédation des couvées par des serpents, des oiseaux tel que le Geai bleu (Cyanocitta

cristata) et des mammifères tel que l’Écureuil roux (Tamiasciurus hudsonicus) (Robinson

2 et al. 1995, Crooks et Soulé 1999, Schmidt 2003), de même que le parasitisme des nids par

le Vacher à tête brune (Molothrus ater), y sont aussi plus marqués (Robinson et al. 1995) et

ce, davantage encore à l’intérieur de matrices résultant de l’agriculture plutôt que de

l’exploitation forestière (Rodewald et Yahner 2001). Certaines espèces, telle que la

Paruline couronnée (Seirus aurocapillus), dites « d’intérieur » (forest-interior specialists,

sensu Whitcomb et al. 1981), semblent tout simplement éviter la proximité des bordures

pour établir leur territoire lors de la nidification (Hunta et al. 1999, Brand et George 2001).

Dans les habitats où la déforestation est marquée au point d’entraîner la fragmentation des

paysages forestiers (Andrén 1994), il va sans dire que ces effets seront d’autant plus

manifestes lorsque les fragments de forêt résiduelle seront petits, non seulement parce que

l’effet de la bordure se fera sentir sur une plus grande proportion de la surface boisée mais

aussi, parce que certaines espèces (area-sensitive species, sensu Freemark et Collins 1992),

telle que le Tangara écarlate (Piranga olivacea), n’y trouveraient pas les grandes surfaces

boisées qu’elles recherchent (Austen et al. 2001).

D’autres mécanismes invoqués pour expliquer les patrons d’abondance et de distribution

des oiseaux forestiers révèlent l’importance directe de la matrice (champs, bûchers, etc.) à

l’intérieur de laquelle se retrouvent les habitats forestiers. Ainsi, le taux d’appariement à

l’intérieur d’îlots boisés est affecté par leur degré d’isolement (Gibbs et Faaborg 1990,

Villlard et al. 1993), les habitats fortement fragmentés réduisant de manière importante la

mobilité des individus (Bélisle et al. 2001, Graham 2001, Gobeil et Villard 2002). Ce type

de résultats a amené la formulation du concept de perméabilité du paysage aux

déplacements des individus (landscape connectivity, sensu Taylor et al. 1993). De plus, il a

été démontré que les individus qui s’aventurent plus loin à l’intérieur des ouvertures

séparant les fragments de forêts sont plus fréquemment victimes des prédateurs aériens

(Hinsley et al. 1995).

Le contexte hivernal La plupart des mécanismes mentionnés plus haut ont été mis en évidence au cours d’études

portant sur des espèces en période de reproduction. Qu’en est-il des mécanismes ayant un

impact sur la dynamique des populations d’oiseaux pendant l’hiver? L’hiver représente un

3 véritable défi pour les oiseaux forestiers des écosystèmes nordiques en ce qui a trait à la

gestion de leurs réserves énergétiques et donc ultimement, à leur survie (Graber et Graber

1979, Perkins et al. 1997). Pendant l’hiver, les températures froides rendent nécessaire la

consommation d’une importante quantité de nourriture. Paradoxalement, l’abondance de la

nourriture se trouve alors à son niveau annuel le plus bas, du moins celle d’origine animale

(insectes, araignées, etc.). De plus, cette situation ne fait guère qu’empirer tout au long

d’une période s’étendant de la fin de l’automne au début du printemps, la quantité de

nourriture disponible ne cessant de diminuer au cours de cette période (soit pendant environ

6 mois à notre latitude). Ce contexte de crise énergétique se trouve exacerbé par le fait

qu’en cette période de l’année, la durée des jours étant grandement réduite, les oiseaux

disposent alors de relativement peu de temps afin de combler leurs besoins énergétiques

courants et pour constituer les réserves endogènes qui leur permettront de survivre au jeûne

nocturne.

Afin de pouvoir maintenir leur équilibre énergétique et survivre dans un tel contexte

environnemental, les espèces résidantes des milieux nordiques ont développé de

remarquables adaptations. Certaines de celles-ci sont comportementales telles que la

constitution de caches de nourriture (Kallander et Smith 1990), la sélection, lors de la quête

alimentaire, de sites exposés au soleil (Carrascal et al. 2001) ou à l’abri du vent (Grubb

1977), la formation de petits groupes compacts (Heinrich 2003) ou l’utilisation de cavités

pendant la nuit (Cooper 1999), les deux dernières adaptations contribuant à réduire de

manière importante les pertes de chaleur par radiation et par convection (Walsberg 1986).

Les adaptations de nature physiologique sont également nombreuses. Celles-ci

comprennent une augmentation de la densité du plumage (Middleton 1986), une

augmentation du métabolisme basal (Broggi et al. 2004), une augmentation de la capacité

thermogénique (Cooper et Swanson 1994), une augmentation de l’endurance

thermogénique (Swanson 1990), une augmentation de la tolérance au froid (Liknes et al.

2002), une diminution de la conductance thermique (Cooper et Gessaman 2004),

l’hypothermie nocturne facultative (McKechnie et Lovegrove 2002) et finalement,

l’accumulation saisonnière et journalière de réserves lipidiques (Pravosudov et Grubb

4 1997a), les acides gras étant le principal substrat utilisé lors de la thermogenèse (Carey et

Dawson 1999).

Un constat s’impose. Malgré l’importance de l’hiver dans le cycle biologique annuel des

d’oiseaux résidants et l’intérêt académique que représente l’étude des phénomènes

spécifiques à cette saison, beaucoup moins de chercheurs se sont intéressés à ce jour aux

effets de la fragmentation des habitats sur l’écologie des espèces résidantes des

écosystèmes tempérés et nordiques, en dehors de la saison de reproduction.

Conséquemment, notre connaissance des effets de la déforestation sur l’abondance, la

répartition spatiale et, notre compréhension des mécanismes affectant la dynamique des

populations d’oiseaux des paysages partiellement déboisés pendant l’hiver sont-elle encore

relativement fragmentaires.

Structure de la thèse La présente étude porte sur les effets de la déforestation sur l’abondance, la répartition

spatiale et les décisions comportementales des oiseaux forestiers pendant l’hiver. Les

travaux d’échantillonnage sur le terrain ont été réalisés pendant trois hivers au Kamouraska,

dans un paysage agro-forestier présentant un gradient complet de déforestation. Certains

aspects des travaux ont porté sur toutes les espèces d’oiseaux présentes dans les habitats

étudiés. Cependant, la majeure partie des efforts a été consacrée à l’étude intensive de

certains aspects de l’écologie de la Mésange à tête noire (Poecile atricapillus). Cette espèce

n’a pas été choisie pour des raisons de vulnérabilité face à la déforestation ou d’un

quelconque statut d’espèce menacée. Bien au contraire, c’est essentiellement en raison de

son abondance et de sa relative ubiquité qu’elle a été choisie comme espèce modèle, du

moins pour la guilde des oiseaux s'alimentant dans les arbres, facilitant ainsi la vérification

de mes hypothèses de recherche. Il aurait en effet été beaucoup plus difficile, du moins

beaucoup plus coûteux, d’entreprendre une telle étude sur une espèce moins abondante

comme la Sittelle à poitrine rousse (Sitta canadensis) ou encore, le Grand Pic (Dryocopus

pileatus).

Je me suis tout d’abord intéressé aux effets de la structure du paysage forestier sur

l’abondance et la répartition spatiale des oiseaux à la fin de l’automne et, au cœur de

5 l’hiver. Le cœur de l’hiver représente une période de l’année où, en raison de la demande

énergétique élevée imposée par les conditions climatiques, la quantité de nourriture

disponible risque de représenter éventuellement, parce que non renouvelable, un facteur

limitant les populations. J’ai donc voulu vérifier expérimentalement si un apport d’énergie

alimentaire illimitée pouvait modifier l’abondance et la répartition spatiale des oiseaux et

ce, à l’intérieur d’un gradient complet de déforestation. Les résultats obtenus et les

interprétations en découlant sont présentés dans le premier chapitre intitulé « Landscape-

dependent distribution of northern forest birds during winter ». Ce premier volet de l’étude

a nécessité l’inventaire des communautés d’oiseaux présentes dans les habitats étudiés. La

réalisation de tels inventaires en dehors de la période de reproduction est problématique

puisque les oiseaux, contrairement à la situation prévalant au début de l’été, sont beaucoup

plus silencieux et se déplacent alors sur des surfaces beaucoup plus grandes que ne le sont

leurs territoires de nidification. J’ai donc eu recours pendant les inventaires à la diffusion

d’un enregistrement de cris de houspillage de la Mésange à tête noire, afin d’augmenter la

probabilité de détection des oiseaux présents dans les paysages étudiés. Les cris de

houspillage de la Mésange à tête noire sont en effet réputés pour le pouvoir d’attraction

qu’ils exercent non seulement sur les membres de cette espèce mais aussi, sur plusieurs

autres espèces d’oiseaux forestiers (Hurd 1996). Cette méthodologie est décrite à l’Annexe

A intitulée « Playbacks of mobbing calls of Black-capped Chickadees help estimate the

abundance of forest birds in winter ».

La répartition spatiale des populations nous renseigne, du moins partiellement, sur la

qualité des habitats. Cependant, qu’en est-il de la condition énergétique des individus

malgré tout présents dans les habitats qui pourraient être présumés de moindre qualité? Les

habitats forestiers fragmentés sont considérés plus coûteux d’un point de vue énergétique

(Hinsley 2000). Je me suis donc à nouveau intéressé aux effets de la structure du paysage

forestier mais cette fois, sur la condition énergétique des Mésanges à tête noire à la fin de

l’automne et, au cœur de l’hiver. J’ai voulu vérifier expérimentalement et ce, toujours à

l’intérieur du même gradient complet de déforestation, si un apport d’énergie alimentaire

illimitée pouvait avoir un effet sur leur condition énergétique mais aussi, s’il pouvait

modifier leur patron journalier d’engraissement des oiseaux présents. Le deuxième chapitre

6 intitulé « Landscape structure, food abundance and winter fattening strategies in black-

capped chickadees » présente les résultats et interprétations découlant de cette expérience.

Dans la mesure où dans les habitats de moindre qualité, la condition énergétique des

Mésanges à tête noire serait affectée ou encore, que le gain énergétique net escompté lors

de la quête alimentaire y serait moindre, j’ai voulu vérifier si au cœur de l’hiver, celles-ci

seraient davantage enclines à s’exposer au risque d’être victimes de prédateurs. J’ai donc

réalisé une expérience visant à évaluer la propension des oiseaux à s’exposer en milieu

ouvert aux prédateurs, dépendamment à nouveau de la sévérité du déboisement et de la

quantité de nourriture disponible. Cette expérience est présentée dans le troisième et dernier

chapitre de la thèse intitulé « Landscape-dependent response to predation risk by forest

birds ».

Chapitre I: Landscape-dependent distribution of northern forest birds in winter

Avertissement Le contenu de ce chapitre a été publié en avril 2005. Hormis quelques changements

mineurs dans le format ayant été nécessaires à la préparation de la thèse, le lecteur trouvera

ici toute l’information contenue dans :

Turcotte, Y, and A. Desrochers. 2005. Landscape-dependent distribution of northern

forest birds in winter. Ecography 28: 129-140.

8

Résumé Nous avons évalué les effets de la structure du paysage le long d’un gradient de

déforestation (couvert forestier variant de 8 à 88 % à l’intérieur d’un rayon de 500 m, n =

24 paysages), sur la répartition spatiale des oiseaux forestiers en hiver, dans le comté de

Kamouraska, Québec. Nous avons de plus conçu une expérience visant à déterminer si

d’éventuels effets persisteraient si, dans la moitié des paysages étudiés (paysages

expérimentaux), une source illimitée d’énergie alimentaire (sous la forme de graines de

tournesol et de gras de bœuf) devenait disponible. Nous avons analysé ces effets au niveau

d’une population de Mésanges à tête noire (Poecile atricapillus), en considérant comme

variable dépendante le nombre d’individus recensés, lors de la diffusion d’un

enregistrement de leurs cris de houspillage, pendant 5 minutes, dans un rayon de 50 m.

Nous avons également analysé ces effets au niveau de la communauté, en nous référant

cette fois à la richesse spécifique. Lors du premier des deux hivers qu’a duré cette étude, en

novembre, avant le début de l’approvisionnement en nourriture, une composante principale

correspondant à l’intégrité des peuplement forestiers (incorporant les variables couvert

forestier et densité des bordures) était positivement associée à l’abondance des Mésanges à

tête noire et à la richesse spécifique. Au cours des deux hivers, de décembre à février,

pendant l’approvisionnement en nourriture, cette composante principale était encore une

fois positivement associée à l’abondance des Mésanges à tête noire et à la richesse

spécifique mais, seulement dans les paysages expérimentaux. Dans les paysages témoins,

l’abondance des Mésanges à tête noire et la richesse spécifique ont au contraire, légèrement

diminué avec une augmentation de la valeur de cette composante principale. Nos résultats

semblent indiquer que les paysages témoins où la déforestation n’était pas marquée ont pu

faciliter l’émigration d’oiseaux lorsque les conditions environnementales (froid, diminution

sous un certain seuil de la quantité de nourriture disponible, diminution de la période de

luminosité) se sont détériorées. Conséquemment, nos résultats suggèrent également que,

dans les paysages témoins où la déforestation était sévère, des oiseaux ont pu se trouver

piégés lorsque les conditions climatiques rigoureuses ont amplifiées les contraintes limitant

déjà leurs mouvements entre les fragments de forêts.

9

Abstract We evaluated the effects of landscape structure, along a broad gradient of deforestation

(forest cover 8–88 %, 500-m radius), on the spatial distribution of forest birds exposed to

winter climatic conditions, in Quebec, Canada. Concurrently, we conducted an experiment

to determine if these effects would persist if an unlimited source of energy, provided by

food-supplementation, became available. We analyzed these effects at the population level,

using count data of black-capped chickadees Poecile atricapillus, but also at the

community level, referring to species richness. In one of the two years of the study, before

food-supplementation began (November), “forest integrity” (a composite of forest cover

and edge density) was positively associated with chickadee abundance and species richness.

Each year, forest integrity was also positively associated with chickadee abundance and

species richness in landscapes that were supplemented (December - February). However, in

control landscapes, during the food-supplementation period, chickadee abundance and

species richness tended to decrease with an increase in forest integrity. We argue that the

more forested control landscapes facilitated winter emigration of juveniles and transient

birds. Conversely, our results further suggest that, in the highly deforested and fragmented

control landscapes, birds became “gap-locked” when rigorous winter climatic conditions

exacerbated already existing movement constraints.

10

Introduction Over the last few decades, many researchers have addressed the impacts of forest loss and,

beyond some threshold, fragmentation (Andrén 1994), on forest bird abundance and species

richness. Stimulated by the persisting theoretical debate about the relative importance of the

ecological processes involved (reviewed by Haila 2002), many empirical studies reported

positive associations of woodlot size, total forest cover or other metrics related to habitat

area, with bird abundance (e.g., Lee et al. 2002), species richness (e.g., Boulinier et al.

2001) or temporal stability in populations and communities (e.g., Hames et al. 2001).

Beyond the direct effects of habitat loss, several mechanisms have been invoked to explain

how changes in landscapes actually impact individuals to result in declines in population

size or species richness larger than expected from habitat loss alone. Thus, processes within

habitat fragments such as reduced food supply (e.g., Burke and Nol 1998), edge avoidance

(e.g., Huhta et al. 1999), increased nest predation (e.g., Crooks and Soulé 1999) and nest

parasitism (e.g., Robinson et al. 1995), have been invoked so far to explain the observed

declines. More rarely, processes occurring in the matrix between habitat fragments have

also been singled out. Thus, reduced pairing success observed in isolated forest patches has

been attributed to the low connectivity (“…the degree to which the landscape facilitates or

impedes movement among resource patches”; Taylor et al. 1993) allowed by severely

deforested landscapes (e.g., Villard et al. 1993).

Most of these proposed mechanisms arose from works conducted during the breeding

season. In fact, relatively few studies have specifically assessed the effects of deforestation

on bird abundance or species richness during the non-breeding season, despite its temporal

and biological importance in the annual cycle. At the higher latitudes particularly, winter

represents a critical energy management challenge: sub-zero temperatures persist for

several months, birds must endure long fasting at night, day length limits their time

available for foraging, and food supply steadily decreases.

11

In this “energy crisis” context, our understanding of the processes responsible for the

spatial distribution and patterns of abundance of birds, in partially deforested landscapes,

remains incomplete. During winter, any changes in population size or species richness in

these landscapes, suboptimal beforehand from an energetic point of view (Hinsley 2000),

would have to result either from mortality, emigration or immigration. Recently, Doherty

and Grubb (2002) reported positive trends between apparent winter survival of some, but

not all, species of forest birds and size of small (0.54 to 6.01 ha; Doherty and Grubb 2000)

isolated woodlots in Ohio. On the other hand, Matthysen (1999) studied a wider gradient of

woodlot sizes and concluded that, in Belgium, yearly survival of nuthatches Sitta europaea

was unrelated to forest size (1 to 1500 ha).

Populations build-up at feeders during winter, far above numbers that could be expected to

come from the immediate surrounding habitats (e.g., Loery and Nichols 1985, Wilson

2001), irruptions (e.g., Bent 1946, Koenig and Knops 2001), and within winter long range

banding recoveries, for many species not even considered migratory (e.g., Wallace 1941,

Browning 1995, Brewer et al. 2000), are well documented. These phenomena illustrate the

importance of emigration and immigration on forest bird population dynamics during

winter. Yet, we do not know to what extent these movements are impeded by the combined

effects of partially deforested landscapes and rigorous winter climatic conditions.

In this study, our objectives were 1) to evaluate the effects of landscape structure, along a

complete gradient of deforestation, on the spatial distribution of forest birds exposed to

severe winter climatic condition, just before, and in the heart of winter, and 2) to determine

experimentally how spatial distribution would be affected if an unlimited source of food

became available, thus compensating, toward one end of this gradient, for an insufficient

continuous habitat area. We analyzed these potential effects at the community level,

referring to species richness, and at the population level, using count data of the black-

capped chickadee Poecile atricapillus.

12

Methods

Study area and experimental design This study was conducted during the winters of 1998-1999 and 1999-2000, from mid-

November through mid-February, on the south-east shore of the St. Lawrence River

estuary, in Kamouraska County (47°30’ N, 69°50’ W), Quebec, Canada (Fig. 1). The study

area covers approximately 600 km2 of agricultural landscape where balsam fir Abies

balsamea, quaking aspen Populus tremuloides, white spruce Picea glauca and paper birch

Betula papyrifera dominate arboreal vegetation. The study area is part of the Temperate

Cold ecoclimatic region (Ecoregion Working Group 1989; Fig. 1). At the La Pocatière

climate station, located within the study area, daily mean temperatures (1971-2000) for

November, December, January, and February are respectively, –0.1°C, –8.3 °C, -11.7 °C,

and –10.3 °C (Anon. 2004).

We selected 24 circular, 500-m radius, and non-overlapping landscapes, centered on a

sharp edge between a field and a forest (Fig. 1). We chose a 500-m radius in order to

include the core of most home ranges of black-capped chickadee winter flocks (10 - 20 ha;

reviewed by Smith 1991) that would occur at the center of the landscapes, while

minimizing the inclusion of habitat beyond their normal flock range. Black-capped

chickadee, our model species, is by far the most abundant permanent-resident forest bird

species in the study area (Turcotte and Desrochers 2002). We established 12 pairs of

adjacent landscapes with similar forest characteristics. The centers of these paired

landscapes were separated from each other by 2.5 –5 km. We chose this distance as we

wanted it large enough to reduce the likelihood that some chickadees would be frequenting

both landscapes over the same winter, while small enough, to guarantee environmental

conditions (microclimate, wild food abundance, predation pressure) as similar as possible

within each pair of landscapes, throughout the study area. Thus, we reduced the risk of

unwanted confounding geographic effects.

13

Figure 1. The study area in Kamouraska County, Quebec, Canada. Circles indicate the 24

500-m radius landscapes. Nonforest habitats are in white, forests within the 24 landscapes

are in black while surrounding forests are in gray, as determined by a LANDSAT-7 satellite

image taken in August 1999. In the insert, ecoclimatic regions in eastern Canada and

localization (asterisk) of the study area.

Temperate Cold

Boreal

LowArctic

Subarctic

St. Law

rence

River

*

14

We provided sunflower seeds and beef suet ad libitum at the center of one landscape of

each pair (hereafter, supplemented landscapes; as opposed to control landscapes) from mid-

November through the end of winter (hereafter, treatment period; as opposed to pre-

treatment period, the first half of November before the beginning of food treatments). The

same food treatments (supplementation or control) were conducted in each landscape

during both winters. Habitations and other feeders than ours were present in the study area.

As we wanted to reduce the likelihood that some birds using control landscape would

nevertheless benefit from human food supplementation, we could not randomize the

assignment of food treatments. Therefore, habitations (and possible food supplementation

from other sources) were indeed present in all supplemented landscapes while the center of

each control landscape was always located at least one kilometer from the nearest

habitation.

Landscape characterization To describe the structure of the 24 landscapes under study, we selected the three landscape

metrics that we considered, referring to both natural history (e.g., Bent 1946) and landscape

ecology (e.g., Andrén 1994, Fahrig 1997), the most likely to explain the distribution of

forest birds. We chose forest cover (%) to quantify the area of potentially suitable habitats

in the landscape for forest birds. Because edges have been shown to affect space use by

forest birds during winter (e.g., Dolby and Grubb 1999, Brotons et al. 2001), we chose edge

density (m/ha of forest) as an index of forest fragmentation. Finally, we used the proportion

of conifers in the forest (%) to provide information about the nature of forested vegetation

as some species may appear or disappear along this gradient (e.g., boreal chickadee P.

hudsonica vs black-capped chickadee) and as we considered that coniferous vegetation

could act as wind-breakers and compensate for the lack of protection against the wind in

the more fragmented landscapes. We used Patch Analyst (Elkie et al. 1999) to obtain these

landscape metrics from a LANDSAT-7 satellite image taken in August 1999. Prior to all

analyses, edge density values were log transformed (hereafter, Ln edge density) while the

proportion of conifers in the forest values were arcsine square-root transformed (hereafter,

Asqrt conifers), in order to approximate normality.

15

We chose not to use popular landscape metrics such as number of forest patches, mean

patch size, or mean distance to nearest patch to characterize our landscapes because most of

the forest cover did not appear in patches isolated from one another. In fact, forested areas

within the 500-m radii were generally in continuity with surrounding forests (Fig. 1).

Bird surveys During winter, forest birds are less vocal and generally dwell over areas larger than their

breeding territories. These factors lessen their probability of detection and would therefore

severely compromise the reliability of standard point counts. Thus, we used playbacks of

mobbing calls of black-capped chickadees to estimate bird abundance during point counts.

The use of this type of playback during winter counts allows the detection of more species

and more individuals (all species combined) than standard point counts (Turcotte and

Desrochers 2002). Furthermore, at least at our latitude (day length at winter solstice is 8 h

28 min; La Pocatière climate station, unpublished data), count results are unaffected by

time of day, whether the number of individuals or species richness are considered (Turcotte

and Desrochers 2002).

We conducted bird surveys at different times of day from sunrise to sunset, once per month

from November through February, at the center of each of the 24 landscapes during both

winters. November surveys were conducted during the pre-treatment period. In November,

migrating forest bird species have already left the study area. None of them return before

March. Calls were playbacked during 5 min with a 5-W amplifier facing skyward and

placed on the ground. Sound level measured one meter above the amplifier with a sound

level meter (RealisticTM) was 105 decibels. We noted all birds seen or heard within a fixed

radius of 50 m. The number of birds we recorded is an index of abundance and should not

be considered otherwise. We did not consider birds flying above the forest canopy as it was

impossible to know if they were users of the landscapes being surveyed. Censuses were not

conducted during heavy precipitation or strong wind.

Additionally, chickadees were captured with mist nets at the center of each of the 24

landscapes in November of both winters, to provide an estimation of the proportion of

juveniles in our study area at the start of each winter. In November, we still were able to

16

age (juvenile or adult) chickadees by the amount of wear on their outermost rectrices (Pyle

1997).

Statistical methods Forest cover and edge density are strongly associated landscape metrics (e.g. Bélisle et al.

2001). Discriminating their respective effects is sometimes practically impossible.

Therefore, we used our three landscape metrics to perform a principal component analysis

(PCA) with Varimax rotation. By doing so, we hoped to obtain a principal component, or

factor, which would amalgamate the information provided by forest cover and Ln edge

density.

Because data were counts (with variance proportional to mean), we used Poisson

regressions to analyze models including the principal components describing landscapes

and food treatment, as predictor variables, and black-capped chickadee abundance or

species richness as response variables. We ran distinct analyses for pre-treatment and

treatment periods. During treatment periods, as each landscape was surveyed once per

month from December through February, count data were averaged to avoid

pseudoreplication (Hurlbert 1984). We carried out statistical analyses with SAS 8.1 (Anon.

1999).

We used an information-theoretic approach (see Burnham and Anderson 2002) for the

interpretation of regression results. We first relied on the coefficient of determination (R2)

to evaluate global model fit. We used this statistic as an indication of whether any of the

models within a set, despite noise and randomness, could represent an acceptable

approximation to an “unknown reality or truth” (Burnham and Anderson 2001). We turned

afterward to the number of estimable parameters (Ki), quasi-likelihood second-order

modification of the Akaike’s information criterion (QAICc), information criterion

difference (∆i), and Akaike weight (wi), to assess the strength of evidence supporting each

of these models. Finally, in order to evaluate the relative importance of the predictor

variables considered, we referred to parameter estimates and unconditional standard errors

obtained by multimodel inference.

17

Results

Landscape structure The 24 landscapes provided a broad gradient of forest cover (8–88 %), proportion of

conifers in the forest (3–66 %), and edge density (65–796 m/ha) (for additional details, see

Turcotte and Desrochers 2003). Landscape metrics within 500-m radii were strongly

correlated with those describing the surrounding landscape, between 500 and 2000 m, a 15

times larger area (forest cover, r = 0.76, p < 0.0001; Ln edge density, r = 0.65, p < 0.001;

Asqrt conifers, r = 0.75, p < 0.0001, n = 24). Thus, the 500-m radius landscapes could be

considered representative of neighboring habitats, which the birds under study, had likely

been using at various degrees. Asqrt conifers was neither correlated with forest cover (r = -

0.004, p = 0.99, n = 24) nor with Ln edge density (r = -0.05, p = 0.8, n = 24). However, as

expected, forest cover and Ln edge density values were strongly and linearly associated (r =

-0.91, p < 0.0001, n = 24; Fig. 2).

4

5

6

7

0 25 50 75 100Forest cover (%)

Ln e

dge

dens

ity

Figure 2. Relationship between forest cover (%) and Ln edge density within a 500-m radius

for the 12 supplemented (filled circles) and the 12 control (open circles) landscapes.

18

Principal component analysis The first principal component (PC1) explained 64% of the variance in the original

landscape metrics data (Table 1). PC1 showed a high positive factor loading for forest

cover and a corresponding high negative factor loading for Ln edge density. We thus

interpret PC1 as a factor not only describing the amount of forested habitat in our

landscapes, but also taking into account their level of fragmentation (hereafter, PC1-Forest

integrity). Thus, while landscapes with the highest and lowest PC1- Forest integrity scores

were, respectively, those with maximal forest cover/minimal edge density values and

minimal forest cover/maximal edge density values, the effect of the level of fragmentation

per se was obviously important in the computation of these scores (e.g. landscape 3C,

forest cover = 48 %, edge density = 379 m/ha, PC1 score = -0.50; landscape 6E, forest

cover = 43 %, edge density = 139 m/ha, PC1 score = 0.19).

Table 1. Landscape metric factor loadings and variance explained by the two principal

components (PC1 and PC2) for the 24 landscapes.

Landscape metric PC1 PC2

Forest cover 0.98 -0.02

Ln edge density -0.98 -0.04

Asqrt conifers 0.01 1.00

Proportion of variance explained 0.64 0.33

A second principal component (PC2) accounted for an additional 33% of the variance

(Table 1). As it had a high coefficient, positive, for Asqrt conifers alone, the interpretation

of this factor is straightforward (hereafter, PC2-Conifers). Thus, PC2- Conifers scores

varied accordingly with Asqrt conifers values, from the less coniferous landscape to the

more coniferous landscape.

19

Landscape structure and use by birds The proximity of habitations in supplemented landscapes could have affected the

distribution and abundance of some species even before we began food treatments.

However, during pre-treatment periods of both winters, we found no difference between

control landscapes and landscapes that would become supplemented, whether we

considered chickadee numbers (winter 1998-1999, control landscapes, mean = 6.5, SE =

1.2, supplemented landscapes, mean = 6.9, SE = 1.4, Wilcoxon two-sampled test, two-

tailed, S = 144, p = 0.7, n = 24; winter 1999-2000, control landscapes, mean = 2.3, SE =

0.6, supplemented landscapes, mean = 2.7, SE = 0.5, S = 142, p = 0.7, n = 24) or species

richness (winter 1998-1999, control landscapes, mean = 1.7, SE = 0.3, supplemented

landscapes, mean = 1.6, SE = 0.3, S = 153, p = 0.9, n = 24; winter 1999-2000, control

landscapes, mean = 0.9, SE = 0.2, supplemented landscapes, mean = 1.3, SE = 0.2, S = 131,

p = 0.3, n = 24).

Furthermore, since counts were conducted near feeders during treatment periods in

supplemented landscapes, the probability of bird detection might have been enhanced.

However, our main objective was not to evaluate the effect of supplementation per se on

forest birds but rather, to determine the effects of landscape structure, and its interaction

with supplementation, on their response. Therefore, we need only to assume that the

sampling bias due to feeders was constant across landscape types.

Playback counts allowed us to detect a total of 18 bird species during the two winters of

this study (Table 2). Of these, seven occurred in at least one third of the 24 landscapes but

only the black-capped chickadee was observed at least once in each landscape (Table 2).

20

Table 2. Number of landscapes (out of the 24 of this study) where each species was

detected (two winters combined), depending on the type of landscape.

Type of landscape

Species Control Supplemented Total

Black-capped chickadee Poecile atricapillus 12 12 24

Common redpoll Carduelis flammea 3 11 14

Downy woodpecker Picoides pubescens 4 8 12

Boreal chickadee Poecile hudsonica 3 7 10

Golden-crowned kinglet Regulus satrapa 5 5 10

Hairy woodpecker Picoides villosus 1 7 8

Blue jay Cyanocitta cristata 2 6 8

Red-breasted nuthatch Sitta canadensis 1 6 7

American goldfinch Carduelis tristis 1 4 5

Pine siskin Carduelis pinus 2 2 4

Pine grosbeak Pinicola enucleator 2 1 3

Gray jay Perisoreus canadensis 0 2 2

White-breasted nuthatch Sitta carolinensis 0 2 2

Brown creeper Certhia americana 0 1 1

Northern shrike Lanius excubitor 0 1 1

Evening grosbeak Coccothraustes vespertinus 0 1 1

Hoary redpoll Carduelis hornemanni 0 1 1

White-winged crossbill Loxia leucoptera 1 0 1

In November, chickadee numbers and population age structure differed markedly between

the two winters of the study (Table 3). November species richness also showed some

21

differences between the two winters of the study (winter 1998-1999, mean = 1.6, SE = 0.2,

winter 1999-2000, mean = 1.1. SE = 0.1, Wilcoxon signed-ranks test, two-tailed, S = 28, p

= 0.08, n = 24). Thus, we conducted separate analyses for each winter. Due to our limited

sample size, the inclusion in our models of year as a main effect, and associated

interactions, would have otherwise resulted in unreliable parameter estimates (Burnham

and Anderson 2002).

Table 3. Mean number of black-capped chickadees detected during playback counts and

total number captured by age. Data obtained at the center of the 24 landscapes in November

for the two winters of the study.

Individuals detected* Individuals captured†

Winter n Mean (SE) Juveniles (%) Adults (%)

1998-1999 24 6.7 (0.9) 51 (61) 33 (39)

1999-2000 24 2.5 (0.4) 48 (22) 172 (78)

*Wilcoxon signed-ranks test, two-tailed, S = 102, p = 0.0002. †Chi-square test, X 2 = 41.9, 1 DF, p < 0.0001.

A larger effort was devoted to capture during the second winter.

In the winter of 1998-1999 during the pre-treatment period, models incorporating only the

predictor variable PC1-Forest integrity were the best to predict chickadee abundance (Table

4) or species richness (Table 5). Accordingly, only PC1-Forest integrity was positively

associated with the number of chickadees detected (Table 6, Fig. 3) or species richness

(Table 7, Fig. 4). However, in the winter of 1999-2000 during the pre-treatment period,

maybe because of the low general abundance of birds at that time, global model fits were

poor and we thus concluded that none of the models within a subset could be considered

good enough, in some absolute sense, to predict either chickadee abundance (Table 4) or

species richness (Table 5). The positive effects of PC1-Forest integrity observed during the

first winter were no longer evident, either for chickadee abundance (Table 6, Fig. 3) or

22

species richness (Table 7, Fig. 4). However, an absence of black-capped chickadee or any

other species was still more likely in landscapes with low PC1-Forest integrity values.

During treatment periods of both winters, the best models were always those with food

treatment, PC1-Forest integrity and their interaction as predictor variables, to predict both

chickadee abundance (Table 4) and species richness (Table 5). Surprisingly, PC1-Forest

integrity had different effects in supplemented and control landscapes, on the abundance of

chickadees (Table 6, Fig. 3) or species richness (Table 7, Fig. 4). Thus, an increase in PC1-

Forest integrity was positively associated with abundance of chickadees or species richness

only in supplemented landscapes while in control landscapes, decreasing trends, though

small, in these response variables were noticeable in both winters. Consequently, chickadee

numbers or species richness in supplemented and control landscapes became increasingly

divergent with an increase in PC1-Forest integrity. Our limited sample size precluded a

month per month (December through February) longitudinal analysis of count data (Stokes

et al. 2000). However, it is worth mentioning that the slope of a linear relationship between

PC1-Forest integrity and chickadee abundance or species richness in the 12 control

landscapes became, and remained, negative in January during the first winter of the study,

and as early as December during the second winter.

23

Table 4. Comparison of Poisson regression models for the association between landscape structure (PC1-Forest integrity, PC2-

Proportion of conifers), food treatment (FT; supplementation or control) and the number of black-capped chickadees detected during

playback counts. Data obtained at the center of the 24 landscapes during pre-treatment (November) and treatment (December -

February) periods for the two winters of the study. Notation for the information-theoretic approach follows Burnham and Anderson

(2002).

Winter Period Predictor variables R2 Kai QAICc ∆i

wi

1998-1999 Pre-treatment PC1, PC2 0.24 4 -98.1 2.3 0.23

PC1 3 -100.4 0.0 0.72

PC2 3 -95.2 5.2 0.05

Treatment FT, PC1, PC2, FT x PC1, FT x PC2 0.52 7 -37.8 7.0 0.03

FT, PC1, FT x PC1 5 -44.8 0.0 0.94

FT, PC2, FT x PC2 5 -38.3 6.6 0.04

24

Table 4 continued.


wi

1999-2000 Pre-treatment PC1, PC2 0.13 4 12.9 2.0 0.22

PC1 3 13.2 2.3 0.19

PC2 3 10.9 0.0 0.59

Treatment FT, PC1, PC2, FT x PC1, FT x PC2 0.73 7 -165.4 6.1 0.04

FT, PC1, FT x PC1 5 -171.4 0.0 0.91

FT, PC2, FT x PC2 5 -165.4 6.0 0.04

a Number of parameter for each model includes the intercept and the estimation from the global model of the variance inflation factor (c).

25

Table 5. Comparison of Poisson regression models for the association between landscape structure (PC1-Forest integrity, PC2-

Proportion of conifers), food treatment (FT; supplementation or control) and species richness during playback counts. Data obtained at

the center of the 24 landscapes during pre-treatment (November) and treatment (December - February) periods for the two winters of

the study. Notation for the information-theoretic approach follows Burnham and Anderson (2002).


wi


PC1 3 43.4 0.0 0.71

PC2 3 47.2 3.8 0.11

Treatment FT, PC1, PC2, FT x PC1, FT x PC2 0.59 7 57.8 7.2 0.02

FT, PC1, FT x PC1 5 50.5 0.0 0.66

FT, PC2, FT x PC2 5 52.0 1.4 0.32

26

Table 5 continued.


wi


PC1 3 54.4 0.0 0.51

PC2 3 55.0 0.6 0.37

Treatment FT, PC1, PC2, FT x PC1, FT x PC2 0.83 7 43.9 6.7 0.02

FT, PC1, FT x PC1 5 37.3 0.0 0.52

FT, PC2, FT x PC2 5 37.5 0.3 0.46

a Number of parameter for each model includes the intercept and the estimation from the global model of the variance inflation factor (c).

27

Table 6. Association between landscape structure (PC1-Forest integrity, PC2-Proportion of conifers), food treatment (supplementation

or control) and the number of black-capped chickadees detected during playback counts. Data obtained at the center of the 24

landscapes during pre-treatment (November) and treatment (December - February) periods for the two winters of the study. Model-

averaged parameters (± unconditional SE) were at first estimated from Poisson regressions.

Winter 1998-1999 Winter 1999-2000

Pre-treatment period Treatment period Pre-treatment period Treatment period

Intercept 1.85 (0.14) 1.53 (0.13) 0.88 (0.16) 2.07 (0.10)

PC1 0.31 (0.13) 0.31 (0.14) 0.06 (0.07) 0.19 (0.10)

PC2 0.03 (0.04) -0.01 (0.02) -0.24 (0.14) 0.02 (0.02)

Food treatment* -0.69 (0.23) -2.34 (0.36)

Food treatment* x PC1 -0.58 (0.23) -0.87 (0.32)

Food treatment* x PC2 0.00 (0.02) 0.01 (0.03)

* Supplementation used as reference category for parameter estimates.

28

Table 7. Association between landscape structure (PC1-Forest integrity, PC2-Proportion of conifers), food treatment (supplementation

or control) and species richness during playback counts. Data obtained at the center of the 24 landscapes during pre-treatment

(November) and treatment (December - February) periods for the two winters of the study. Model-averaged parameters (±

unconditional SE) were at first estimated from Poisson regressions.

Winter 1998-1999 Winter 1999-2000

Pre-treatment period Treatment period Pre-treatment period Treatment period

Intercept 0.44 (0.14) 0.66 (0.12) 0.07 (0.13) 0.94 (0.09)

PC1 0.29 (0.12) 0.20 (0.08) 0.10 (0.08) 0.09 (0.05)

PC2 0.02 (0.04) 0.08 (0.08) 0.02 (0.07) 0.08 (0.08)

Food treatment* -0.72 (0.21) -1.30 (0.19)

Food treatment* x PC1 -0.29 (0.14) -0.23 (0.10)

Food treatment* x PC2 -0.11 (0.09) 0.04 (0.09)

* Supplementation used as reference category for parameter estimate.

29

Pre-treatment period

0

4

8

12

16

-2 0 2

PC1-Forest integrity

Num

ber o

f chi

ckad

ees Winter 1998-1999

0

4

8

12

16

-2 0 2


Num

ber o

f chi

ckad


Treatment period

0

4

8

12

16

-2 0 2


Num

ber o

f chi

ckad


0

4

8

12

16

-2 0 2


Num

ber o

f chi

ckad


Figure 3. Relationship between forest integrity and number of black-capped chickadees

detected during playback counts. Data obtained at the center of the 12 supplemented (filled

circles) and the 12 control (open circles) landscapes for the two winters of the study.

During pre-treatment periods (November), all landscapes were still non-supplemented.

Number of chickadees during treatment periods (December - February) represents the mean

of three monthly counts per landscape. The y axes were held constant to emphasize yearly

variation in chickadee abundance. When zero was excluded from 95 % unconditional

confidence interval of parameter estimates (effect size > 0), curves show predicted numbers

of chickadees derived from Poisson regressions.

30

Pre-treatment period

0

1

2

3

4

-2 0 2


Spe

cies

rich

ness

Winter 1998-1999

0

1

2

3

4

-2 0 2


Spe

cies

rich

ness

Winter 1999-2000

Treatment period

0

1

2

3

4

-2 0 2


Spe

cies

rich

ness

Winter 1998-1999

0

1

2

3

4

-2 0 2


Spe

cies

rich

ness

Winter 1999-2000

Figure 4. Relationship between forest integrity and species richness during playback

counts. Data obtained at the center of the 12 supplemented (filled circles) and the 12 control

(open circles) landscapes for the two winters of the study. During pre-treatment periods

(November), all landscapes were still non-supplemented. Species richness during treatment

periods (December - February) represents the mean of three monthly counts per landscape.

The y axes were held constant to emphasize yearly variation in species richness. When zero

was excluded from 95 % unconditional confidence interval of parameter estimates (effect

size > 0), curves show predicted species richness derived from Poisson regressions.

31

Discussion Survey results for the pre-treatment period of the first winter of the study, and in

supplemented landscapes during treatment periods of both winters, all conform to most

previously published studies conducted during the non-breeding season, reporting positive

associations between habitat area (corresponding here to forest integrity), bird abundance

(here chickadees), or species richness (e.g., Blake 1987, Tellaria and Santos 1995, Doherty

and Grubb 2000).

However, the positive association between PC1-Forest integrity values and black-capped

chickadee numbers or species richness in supplemented landscapes during treatment

periods could also be explained by landscape matrix effects, rather than only by mere

habitat area effects per se. Resident bird populations are usually composed of adult birds

with high site fidelity, already present during the breeding season, but also of dispersing

first-year individuals born elsewhere (reviewed by Matthysen 1993). In the black-capped

chickadee, dispersal can indeed occur throughout the winter (Weise and Meyer 1979). The

presence of transient (or floater) black-capped chickadees not permanently associated to a

particular home range in winter has also been documented (Smith 1984). Thus, survey

results in supplemented landscapes may indicate that, in the more deforested landscapes,

despite the energetic enhancement provided by food-supplementation, gaps represented

movement constraints impeding the discovery and an eventual use of the center of these

landscapes by black-capped chickadees, and by other species as well (as indicated by the

similar effect on species richness), coming from outside the 500-m radius.

Results in control landscapes for treatment periods of both winters offer some support to

the landscape matrix effects hypothesis. In contrast to pre-treatment period and

supplemented landscapes treatment period results, they consistently show for each winter,

decreasing, though small, trends in black-capped chickadee numbers or species richness

with increasing PC1-Forest integrity values. This suggests that, in the less deforested

control landscapes, birds had more opportunities to explore neighboring habitats, but

furthermore, that some individuals emigrated to, more or less distant, more favorable

locations. If we assume that birds are able to maximize their survival throughout the winter

by some Bayesian updating process (McNamara and Houston 1980) of habitat selection

32

(reviewed by Block and Brennan 1993), then, many juveniles and transients could have

permanently left highly forested control landscapes, when climatic conditions became

harsher (during both winters of the study, there were 14 days during which the minimum

temperature went below -20°C in our study area; La Pocatière climate station, unpublished

data) and expected non-renewable food supply fell below some threshold. Similarly,

Doherty and Grubb (2002) postulated that the lower apparent survival rates of first-year

Carolina chickadees P. carolinensis they observed in forested river corridors, when

compared to rates in their isolated woodlots, resulted from the on-going winter dispersal of

first-year birds.

In the more deforested control landscapes, movement constraints may have left birds “gap-

locked”. “Gap-locking" or the reduced ability to leave patches due to isolation has received

little attention compared to constraints on the ability to reach patches. Yet, when climatic

conditions deteriorate and days become shorter, exploration or emigration likely becomes

too hazardous. Indeed, Grubb and Doherty (1999) found that the median distance of gaps

crossed decreases from fall to late winter in the resident community they studied in Ohio.

Even in the absence of the exacerbating effect of harsh climatic conditions, many studies

reported that gaps and isolation represent movement constraints in forested landscapes

(e.g., Bélisle et al. 2001, Cooper and Walters 2002). These constraints have also been

singled out as the cause of an unusually high abundance of birds in small forest fragments

(Zanette 2001).

The relatively more important disappearance of black-capped chickadees as well as the loss

of species in control landscapes with high PC1-Forest integrity values could also have been

interpreted as increased mortality. We consider this possibility very unlikely. Presumably,

less deforested landscapes maximize the survival of birds as they offer them better

protection against winds in a larger proportion of their home ranges (Dolby and Grubb

1999). Both foraging (Grubb 1977) and hoarding (Brotons et al. 2001) behaviors are indeed

affected by this abiotic edge effect. Also, landscapes where forest cover is important offer

access to roosting and feeding sites with few gaps to cross and therein, reduce the exposure

to aerial predators such as the northern shrike Lanius excubitor or the northern goshawk

Accipiter gentilis, both present in our study area. But what do we know for sure based on

33

previously published studies, about the relative impacts of winter mortality and emigration

on forest birds? Some studies conducted in winter concluded to higher survival rates in

supplemented than in control birds (e.g., Brittingham and Temple 1988), in dominants than

in subordinates (Desrochers et al. 1988), and in adult than in first-year individuals (e.g.,

Lahti et al. 1998). As rightly acknowledged by Loery et al. (1987) and Karr et al. (1990), it

is impossible, even with marked birds, to distinguish winter mortality and emigration rates

in these populations, particularly for first-year dispersing birds. Survivorship rates obtained

in this context, even if they take into account recapture/resighting probabilities, are

nevertheless, a composite value integrating both mortality and emigration in proportions

impossible to define.

To conclude, we did not demonstrate beyond any doubt that emigration occurred because,

as it is the case in studies studying mortality, it is impossible to know the fate of

disappearing birds in an open system like ours. However, our data suggest that differences

in control landscape matrices were responsible for the variation in black-capped chickadee

abundance, species richness, and patterns of spatial distribution we observed. Maybe the

broad gradient of deforestation our study area allowed and the severity of local winter

conditions have helped reveal this process. The counterintuitive results we obtained do not

support the common assumption that mortality is the main factor behind birds’

disappearance during winter. Thus, beyond the already recognized importance of large

unfragmented forest patches as suitable habitats per se for forest birds in winter, large

forest tracts appear to facilitate winter movements of at least, part of the population (e.g.,

subordinate juveniles). Populations of common winter residents such as black-capped

chickadees may be affected only marginally by constraints to movements like those

discussed above. However, those constraints may prove detrimental to populations of

regionally uncommon species such as boreal chickadee and brown creeper Certhia

americana.

Chapitre II: Landscape structure, food abundance and winter fattening strategies of black- capped chickadees

Avertissement Le contenu de ce chapitre sera soumis d’ici la fin de 2005 à une revue de recherche en

écologie animale. Hormis quelques changements mineurs dans le format ayant été

nécessaires à la préparation de la thèse, le lecteur trouvera ici toute l’information

contenue dans le manuscrit qui sera soumis.

35

Résumé L’hiver représente un défi considérable pour les oiseaux diurnes de petite taille.

L’accumulation de réserves de graisse pendant la journée doit être suffisante afin de leur

permettre de survivre au jeûne nocturne et parfois même au-delà, si les conditions

météorologiques au moment du lever du soleil sont défavorables. D’après les modèles

théoriques, les oiseaux devraient cependant toujours chercher à minimiser leur masse, à

moins que le risque de mort par inanition ne devienne trop élevé. Les stratégies

journalières d’engraissement chez les oiseaux ont suscité à ce jour l’intérêt d’un grand

nombre de chercheurs. Cependant, cette problématique n’a pas encore été placée dans le

contexte d’oiseaux soumis à des conditions climatiques rigoureuses dans des habitats

partiellement déboisés. Le but de cette étude était de vérifier si la structure du paysage et

l’abondance de la nourriture avaient un effet sur la condition énergétique et les patrons

quotidiens d’engraissement des oiseaux pendant l’hiver, par le biais d’une expérience

d’approvisionnement en nourriture le long d’un gradient complet de déforestation. J’ai

choisi la Mésange à tête noire Poecile atricapillus (Linnaeus), une espèce se constituant

des caches de nourriture, en guise d’organisme modèle. La structure du paysage n’a eu

aucun effet sur la condition énergétique des individus contrairement à

l’approvisionnement en nourriture. Ce dernier résultat suggère qu’en raison du caractère

stochastique des conditions météorologiques, les oiseaux bénéficiant de

l’approvisionnement ont choisi d’en tirer profit, engraissant à un rythme constant tout au

long de la journée, comme si le risque de mort par inanition était perçu comme étant plus

immédiat que le risque de prédation. L’approvisionnement en nourriture et la progression

de l’hiver ont eu un effet sur le patron quotidien d’engraissement. Dans les conditions

plus clémentes du début de l’hiver, les oiseaux engraissaient à un rythme constant. Au

coeur de l’hiver, les mésanges témoins, contrairement à celles bénéficiant de

l’approvisionnement, engraissaient beaucoup plus rapidement pendant la deuxième

moitié de la journée, minimisant ainsi les coûts associés à une masse trop élevée pendant

la première moitié de la journée, sans pour autant compromettre leurs chances d’être

suffisamment grasses avant la tombée de la nuit.

36

Abstract At the higher latitudes, winter represents a critical energy management challenge for

small diurnal birds. The daily build up of fat reserves must allow them to survive the

fasting of the following night, and to withstand possible inclement weather conditions

which might disrupt foraging at dawn. Theory predicts that, because of predation risks,

birds should minimize their mass unless starvation risks become too high. Many

modellers and empiricists have addressed winter fattening strategies in small birds.

However, studies published so far have not been placed into a landscape context, despite

well-documented effects of landscape structure on key aspects of the ecology of birds

exposed to severe winters. Here, we investigate whether landscape structure and food

abundance affect the energetic condition and the pattern of daily fattening in a population

of wild birds during winter. We conducted a food-supplementation experiment repeated

along a complete gradient of deforestation, using mass corrected for size and fat score

data for the black-capped chickadee Poecile atricapillus (Linnaeus), a small food-

hoarding passerine. Landscape structure did not affect chickadees’ energetic condition.

However, food-supplementation improved both surrogate measures of total body fat

level. The latter result suggests that in response to weather unpredictability, they took

advantage of supplementation, gaining fat at a constant rate, as if they perceived

starvation to represent a more proximate risk than predation. Food-supplementation and

winter progression affected the pattern of daily fattening. In the milder conditions of early

winter, before the beginning of supplementation, chickadees gained fat at a constant rate.

However, later during winter, control chickadees, contrary to supplemented chickadees,

delayed most of their fattening toward the last half of the day, minimizing mass

associated costs during the first half of the day, without compromising their chances of

being fat enough at dusk.

37

Introduction At the higher latitudes or at high elevations, winter represents a critical energy

management challenge for endotherms. This challenge is particularly acute for small

diurnal birds for several reasons. Flight, when compared to walking, is costly in terms of

energy expenditure (Berger & Hart 1974; Pennycuick 1989). Moreover, their high surface

area to volume ratio decreases heat conservation (Calder 1984; Ahlborn & Blake 2001).

In temperate and boreal ecosystems, sub-zero temperatures may persist almost without

interruption for months, food supply steadily decreases, day length greatly limits time

available for foraging, and they must endure long fasting at night. They further have to

cope with a marked unpredictability of resource access because snow and freezing rain

storms periodically compromise movements, or make extensive parts of their foraging

substrate inaccessible.

During winter, the daily build up of fat reserves (reviewed by Blem 1990) must allow

small birds to survive the fasting of the following night, and to withstand possible

inclement weather conditions which might disrupt foraging at dawn. Small birds must

sometimes forage intensively to reach sufficient fattiness (7-15 % of fat-free body mass

in passerines; Lehikoinen 1987). However, getting fat is costly. Foraging has been shown

to decrease vigilance (Caraco 1979) and to increase exposure to predators (Lima 1986).

Furthermore, the resulting mass gain increases mass-dependent energy expenditure which

in turn increases foraging time necessary for maintenance of a higher level of fat

(McNamara & Houston 1990 but see McNamara, Ekman & Houston 2004). Moreover,

mass gain increases wing load (Pennycuick 1989), impairs flight ability to escape from

predator attacks (Krams 2002), and hence presumably, above some threshold, increases

predation risk (Metcalfe & Ure 1995). Therefore, according to the “mass-dependent

predation hypothesis”, birds should minimize their mass in order to minimize predation

risk, unless starvation risks become too high (Witter & Cuthill 1993). Indeed, even during

winter, birds maintain fat levels below their physiological capacity (Pravosudov & Grubb

1997a). Thus, fat levels are considered to represent a trade-off between starvation and

predation risks that would maximize survival probability (Lima 1985 but see Pravosudov

& Grubb 1998a). It follows that the amount of fat carried by a bird at any moment during

38

the day should reflect the relative importance of associated costs and benefits, as a

function of time available before sunset, expected rate of gain, and current and

foreseeable climatic conditions.

Over the last two decades, many researchers have addressed winter daily foraging

routines, fattening, and the external energy storage strategy of food hoarding in small

birds, both from theoretical (e.g. Houston & McNamara 1993; Clark & Ekman 1995;

Brodin & Clark 1997; Pravosudov & Lucas 2001) and empirical (e.g. Pravosudov &

Grubb 1997b; Lilliendahl 2002; Koivula, Orell & Lahti 2002; Macleod et al. 2005)

perspectives. However, models and field studies published so far have not been placed

into a landscape context, despite reported effects of landscape structure (patch size,

isolation, etc.) on key aspects of the ecology of birds during the non-breeding season,

particularly abundance (e.g. Blake 1987 but see Hamel, Smith & Wahl 1993), survival

(Doherty & Grubb 2002 but see Matthysen 1999), emigration constraints and spatial

distribution (Turcotte & Desrochers 2005), and anti-predator behaviour (Tellaria et al.

2001; Turcotte & Desrochers 2003).

Here, we investigate whether landscape structure and food abundance affect the fattening

strategy of black-capped chickadee Poecile atricapillus (Linnaeus), a small (10-14 g;

Smith 1991) food-hoarding passerine (e.g. Brotons, Desrochers & Turcotte 2001),

exposed to severe winters. Specifically, we measure how their energetic condition and

pattern of daily fattening respond to a food-supplementation experiment repeated along a

complete gradient of deforestation. We test the following predictions:

1. Because fragmented forests are presumed costly from an energetic point of view

(Hinsley 2000), deforestation would lower body mass index and fat score values in birds.

2. Assuming that natural food supply is limiting black-capped chickadee populations

during winter (Brittingham & Temple 1988), this effect of deforestation would not be

present with food supplementation.

3. Because of the severe climatic conditions prevailing in our study system (requiring

birds to be relatively fat at dusk), we would observe the appearance over the winter of the

39

pattern of delayed daily fattening modeled by McNamara, Houston & Krebs (1990) for

hoarding species having to trade-off starvation risk and mass associated costs (predation

risk and higher energy expenditure).

4. Assuming that birds are able to adjust their feeding decisions and optimize their body

reserves according to predictors of foraging uncertainty (e.g. Rogers & Smith 1993;

Macleod et al. 2005), the predictability in energy gain provided by food supplementation

would further delay the daily fattening, reflecting a delayed shift in the relative

importance of mass associated costs and starvation risk.

40

Materials and methods

Study area and experimental design This study was conducted during the winters of 1998-1999 and 1999-2000, from

November through March, on the south-east shore of the St. Lawrence River estuary, in

Kamouraska County (47°30’ N, 69°50’ W), Quebec, Canada. The study area covers

approximately 600 km2 of agricultural landscape where balsam fir Abies balsamea

(Linnaeus), quaking aspen Populus tremuloides (Michaux), white spruce Picea glauca

(Moench), and paper birch Betula papyrifera (Marshall) dominate arboreal vegetation. It

is part of the temperate cold ecoclimatic region (Ecoregion Working Group 1989). At the

La Pocatière climate station, located within the study area, temperatures may get as low

as -30°C during winter (Environment Canada 2005).

We selected 24 circular, 500-m radius, and non-overlapping landscapes, centered on a

sharp edge between a field and a forest. A 500-m radius was chosen in order to include

the core of most home ranges of black-capped chickadee winter flocks (10 - 20 ha;

reviewed by Smith 1991) that would occur at the center of the landscapes, while

minimizing the inclusion of habitat beyond their normal flock range. We established 12

pairs of adjacent landscapes with similar forest characteristics. The centers of these paired

landscapes were separated from each other by 2.5 –5 km. This distance was chosen as we

considered it large enough to reduce the likelihood that some individuals would occur in

both landscapes (see Smith 1991), while small enough to provide environmental

conditions (microclimate, wild food abundance, predation pressure) as similar as possible

within each pair of landscapes, throughout the study area. Sunflower seeds and beef suet

were provided ad libitum at the center of one landscape of each pair (hereafter,

supplemented landscapes; as opposed to control landscapes) from the last half of

November through the end of March (hereafter, treatment period; as opposed to pre-

treatment period, November, before the beginning of food treatments). By this spatial

interspersion of food treatments (supplementation or control) throughout the study area,

we intended to eliminate the risk of confounding geographic effects. Habitations were

present in the study area. As we aimed to reduce the likelihood that in control landscapes,

41

some birds would nevertheless benefit from food supplementation, we could not

randomize the assignment of food treatments within each pair. Thus, while habitations

(and eventual possible food supplementation from other sources) were indeed present in

supplemented landscapes, the center of each control landscape was conversely always

located at least one kilometer from the nearest habitation.

Landscape characterization To describe the structure of the 24 landscapes under study, we used Patch Analyst (Elkie

et al. 1999) to obtain, from a LANDSAT-7 satellite image taken in August 1999, the

three landscape metrics that we considered, referring to both natural history (Bent 1946)

and landscape ecology (Andrén 1994; Fahrig 1997) literature, the most likely to affect the

energetic condition and patterns of daily fattening in black-capped chickadees. We chose

forest cover (%) to quantify the area of potentially suitable habitats in the landscapes.

Because edges have been shown to affect space use and foraging by forest birds during

winter (Dolby & Grubb 1999; Brotons et al. 2001), we chose edge density (m/ha of

forest) as an additional index of forest fragmentation. Finally, we used the proportion of

conifers in the forest (%) to provide information about the nature of forested vegetation

as we considered that, in the more fragmented landscapes, coniferous vegetation could

act as wind-breakers and compensate for the lack of protection against the wind, an

important cause of heat loss (Thompson & Fritzell 1988).

The 24 landscapes under study provide a broad gradient of forest cover (8–88 %),

proportion of conifers in the forest (3–66 %), and edge density (65–796 m/ha) (Turcotte

& Desrochers 2003). In an earlier study, we performed a principal component analysis

with these landscape metrics (for additional details, see Turcotte & Desrochers 2005). We

obtained a first principal component describing the amount of forested habitat in our

landscapes while taking into account their level of fragmentation (hereafter, PC1-Forest

integrity), and a second principal component describing the relative abundance of

conifers (hereafter, PC2-Proportion of conifers). In the present study, these two principal

components are used as predictor variables describing the structure of our landscapes.

42

Trapping and measurements Chickadees were captured with mist nets at the center of each landscape, throughout the

daylight period, during pre-treatment (November) and treatment periods (January-March)

of both winters. When captures started during the treatment period, food treatments were

already in their second month. The presence of feeders in supplemented landscapes

during the treatment period was sufficient to attract and capture birds. During the pre-

treatment period, and in control landscapes during the treatment period, we used

playbacks of mobbing calls of black-capped chickadees (Turcotte & Desrochers 2002) to

attract birds toward the nets. Reluctance to fly toward playbacks could have induced

potential sex, age or condition bias. We consider this possibility unlikely as it has been

demonstrated that playbacks of mobbing calls do not affect the perception of predation

risk by black-capped chickadees under cover (Desrochers, Bélisle & Bourque 2002).

The capture time for each bird was recorded to the nearest minute. All birds were banded

with a U.S. Fish and Wildlife Service aluminium band to allow the identification of

individuals captured on more than one occasion. During the pre-treatment period, age

could be determined by the amount of wear on their outermost rectrices (Pyle 1997).

All measurements were taken by one of us (YT) thus eliminating among-observers

variability which potentially, could obscure patterns of natural variability. Wing lengths

(chord) were recorded to the nearest 0.5 mm with a stopped steel rule. As we often

observed bent wings due to cavity roosting, only the longest wing value for each bird was

considered. Tarsus length is a relatively difficult measurement to perform on live birds

with calipers (Pyle 1997). Thus, tarsus lengths were also measured to the nearest 0.5 mm

with a stopped steel rule. The tibiotarsus-tarsometatarsus articulation was then held

against the end-stop of the rule and the length was read at the level of the last scale leg

scale before the toes emerge. This unconventional method results in slightly longer

readings than when tarsus length is evaluated with callipers from the depression in the

angle of the intertarsal joint and the distal end of the last leg scale before the toes emerge.

Though the use of a stopped steel rule does not offer as much accuracy (the closeness of a

measured value to its true value; Sokal & Rohlf 1981) as the caliper method, it is much

easier and faster to perform in the field on small birds, and we found values obtained in

43

this manner were more repeatable. Tarsus length measurements were averaged to provide

a mean tarsus length for each bird thus reducing the impact of potential measurement

errors. Finally, body mass was determined to the nearest 0.1 g with an electronic scale.

Evaluation of energetic condition Energetic (or body or nutritional) condition (or state or status) was evaluated with two of

the most frequently used, non-invasive techniques applicable to small birds as surrogate

measures of total body fat level. We used estimation of the amount of visible

subcutaneous fat deposits in the tracheal pit (or furcula or interclavicular depression)

(hereafter, fat score), and mass corrected for structural size (hereafter, body mass index)

as response variables. By taking these two different approaches, we wanted to strengthen

the empirical evidence, or not thereof, that would provide constant results.

Fat score (or class) was evaluated immediately following capture, to avoid subjectivity

that could have resulted otherwise if preceded by structural size or mass measurements.

We used the classification of Gosler (1996) in which scores range from 0 (no visible fat

in the tracheal pit) to a maximum of 5 (fat filling tracheal pit, bulging and overlying

pectoral muscle). Fat score was found to be linearly related to claviculo-coracoid fat mass

(Redfern et al. 2000), total body fat reserves (Blem 1990 but see Kaiser 1993) and total

body mass (Kullberg, Jakobsson & Fransson 2000) in some species of passerines.

However, to our knowledge, no such data exist for the black-capped chickadee.

We consider body mass index to represent a more sensitive (since not restrained to a

limited number of classes) and more objective procedure (particularly when the visible

amount of fat is near the limit of two adjacent classes) than the evaluation of fat score.

However, the use of body mass index has been criticized because it must be assumed that

a given size character varies isometrically with body size (Blem 1984). This represents an

unlikely issue because, due to strong natural selection against nonfunctional relative

sizes, scaling relationships between appendages and body size show low intraspecific

variation around average allometries (Frankino et al. 2005). Nevertheless, it is

noteworthy that body mass index cannot tease apart the contributions of fat and

undigested gut contents to body mass.

44

In many published studies, body mass index is obtained by dividing mass by one size

character raised to the third power (e.g. Winker, Warner & Weisbrod 1992; Yong &

Moore 1993; Dunn 2001). However, such cubic transformations are not recommended

(Greenwood 2003). Winker (1995) found indeed that, despite the volumetric nature of

mass, cubed size character values were less accurate than unmodified values to predict

actual fat content of Tennessee warblers Vermivora peregrina. Therefore, we used as

others (e.g. Winker 1995; Merom, Yom-Tov & McClery 2000; Dunn 2002), an

untransformed size character, and calculated body mass index by dividing body mass by

mean tarsus length (BMI = 10 X Mass/Tarsus). We preferred tarsus length to wing length

as an indicator of structural size because, as in most passerines (Pyle 1997), juvenile

primaries were shorter than adult primaries in this chickadee population (Fig. 1). Using

wing length would have therefore systematically overestimated condition of shorter

winged juveniles, all other factors (skeleton size, fat mass, body mass) being equal.

Furthermore, the use of skeletal features is considered preferable because wing length

also varies with physical wear (Brown 1996).

58.0

61.0

64.0

67.0

70.0

16.0 17.0 18.0 19.0 20.0

Mean tarsus length (mm)

Long

est w

ing

leng

th (m

m)

Figure 1. Relationship between mean tarsus length and longest wing length of 212 black-

capped chickadees of known age. Open circles and thin line represent 79 juveniles, and

filled circles and bold line represent 133 adults.

45

Statistical analysis Energetic condition.-Regressions with normal or binomial error distributions were used

to analyze models including the two principal components describing landscapes, food

treatment, and time elapsed since sunrise at the time of capture (expressed as a percentage

of total day length between sunrise and sunset; La Pocatière climate station, unpublished

data) as predictor variables, and respectively, body mass index or fat score as response

variables. Distinct analyses were run for pre-treatment and treatment periods but data of

both winters were pooled because of otherwise insufficient sample sizes in control

landscapes.

The debate persists about the proper way of analyzing fat scores (e.g. Brown 1996;

Rogers 2003). We decided to consider them as ordered categorical data. We first

conducted a polytomous response (fat scores 0-1 or 3 or 4 or 5) logistic regression for the

treatment period but had to content ourselves with a dichotomous response (fat scores 1-2

or 3-4-5) logistic regression for the pre-treatment period because, in this case, data did

not meet the proportional odds assumption (Stokes, Davis & Koch 2000). This

assumption meant that, taking into account the full model of predictor variables, the

likelihood of passing from one fat score category to the next had to be constant. All

statistical analyses were carried out with SAS 8.1 (SAS Institute Inc. 1999).

We adopted an information-theoretic approach (see Burnham & Anderson 2002;

Stephens et al. 2005) for the interpretation of regression results. We first assessed

goodness-of-fit of global models relying on the coefficient of determination (R2), the

Hosmer and Lemeshow statistic or the Pearson statistic for, respectively, normal

regressions, dichotomous response and polytomous response logistic regressions. These

statistics were used as an indication of whether any of the models within a set, despite

noise and randomness, could represent an acceptable approximation to an “unknown

reality or truth” (Burnham & Anderson 2001). We turned afterward to the number of

estimable parameters (Ki), second-order version of the Akaike’s information criterion

(AICc), information criterion difference (∆i), and Akaike weight (wi), to assess the

strength of evidence supporting each of these models. Finally, in order to evaluate the

46

relative importance of the predictor variables considered, we referred to parameter

estimates and unconditional standard errors obtained by multimodel inference. Not all

possible candidate models were used in the analyses. Based on the existing literature

reporting the effect of time of day on the mass of northern birds during winter (e.g.,

Graedel & Loveland 1995; Koivula et al. 2002), all models considered here included,

time elapsed since sunrise at the time of capture, in combination with one or more of the

other predictor variables and relevant interaction terms.

Patterns of daily fattening.-Patterns of daily fattening were addressed with tests for

quadratic effect from body mass index data.

47

Results For birds captured on more than one occasion during this study (57 individuals: most of

them in supplemented landscapes, during the pre-treatment period and later during the

treatment period), only one randomly selected observation was considered in the analysis

to avoid pseudoreplication (Hurlbert 1984). This represents 161 birds for the pre-

treatment period and for the treatment period, 64 and 486 birds, respectively, in control

and supplemented landscapes. These 711 birds captured throughout the daylight period

are considered independent sampling units of the population of all birds present in the 24

landscapes under study.

Energetic condition Based on goodness-of-fit statistics, we considered that normal (pre-treatment period: R2 =

0.12; treatment period: R2 = 0.20) and logistic (pre-treatment period: Hosmer and

Lemeshow, P = 0.23; treatment period: Pearson, P = 0.88) regression models did not lack

fit. During the pre-treatment period, all candidate models were closely equivalent to

predict body mass index (Table 1) or fat score (Table 2) as indicated by the low

information criterion differences (∆i). Time elapsed since sunrise was the only predictor

variable associated (positively) with both body mass index (Table 3) and fat score (Table

4). Before running normal regressions for the treatment period, body mass index values

were log-transformed (hereafter, log body mass index) to homogenize residual variance.

During the treatment period, the best models all included food treatment and time elapsed

since sunrise to predict log body mass index (Table 1) or fat score (Table 2).

Accordingly, food treatment and time elapsed since sunrise were the only predictor

variables associated (positively) with both log body mass index (Table 3) and fat score

(Table 4). For tables readability, three-way interaction terms (principal components

describing landscapes structure X food treatment X time elapsed since sunrise) for which

zero was included within 95% unconditional confidence interval of parameter estimates

(effect size ≤ 0), and intercepts, are not shown.

48

Table 1. Comparison of normal regression models for the association between landscape structure (PC1-Forest integrity, PC2-

Proportion of conifers), food treatment (FT; supplementation or control), time elapsed since sunrise (TS; proportion (%) of total day

length) and body mass index (pre-treatment period) or log body mass index (treatment period) of black-capped chickadees. Data

obtained at the center of the 24 landscapes during pre-treatment (November) and treatment (January-March) periods. Notation for the

information-theoretic approach follows Burnham and Anderson (2002).

Period Predictor variables Kai AICc ∆i

wi

Pre-treatment PC1, PC2, TS, PC1 x TS, PC2 x TS 7 108.9 2.0 0.19

PC1, TS, PC1 x TS 5 108.0 1.2 0.29

PC2, TS, PC2 x TS 5 106.9 0 0.52

Treatment FT, PC1, PC2, TS, FT x TS, PC1 x TS, PC2 x TS, FT x PC1 x TS, FT x PC2 x TS 11 -2542.1 0.1 0.44

FT, PC1, TS, FT x TS, PC1 x TS, FT x PC1 x TS 8 -2534.8 7.5 0.01

FT, PC2, TS, FT x TS, PC2 x TS, FT x PC2 x TS 8 -2542.2 0 0.46

PC1, PC2, TS, PC1 x TS, PC2 x TS 7 -2534.9 7.3 0.01

FT, TS, FT x TS 5 -2536.5 5.8 0.03

PC1, TS, PC1 x TS 5 -2536.7 5.6 0.03

PC2, TS, PC2 x TS 5 -2536.1 6.2 0.02

a Number of parameter for each model includes the intercept and the residual variance.

49

Table 2. Comparison of logistic regression models for the association between landscape structure (PC1-Forest integrity, PC2-

Proportion of conifers), food treatment (FT; supplementation or control), time elapsed since sunrise (TS; proportion (%) of total day

length) and fat score of black-capped chickadees. Data obtained at the center of the 24 landscapes during pre-treatment (November)

and treatment (January-March) periods. Notation for the information-theoretic approach follows Burnham and Anderson (2002).

Period Predictor variables Kai AICc ∆i

wi

Pre-treatment PC1, PC2, TS, PC1 x TS, PC2 x TS 6 137.8 1.7 0.18

PC1, TS, PC1 x TS 4 136.1 0 0.43

PC2, TS, PC2 x TS 4 136.3 0.2 0.39

Treatment FT, PC1, PC2, TS, FT x TS, PC1 x TS, PC2 x TS, FT x PC1 x TS, FT x PC2 x TS 10 1807.2 110.1 0

FT, PC1, TS, FT x TS, PC1 x TS, FT x PC1 x TS 7 1799.8 102.7 0

FT, PC2, TS, FT x TS, PC2 x TS, FT x PC2 x TS 7 1800.4 103.2 0

PC1, PC2, TS, PC1 x TS, PC2 x TS 6 1793.1 96.0 0

FT, TS, FT x TS 4 1697.1 0 1

PC1, TS, PC1 x TS 4 1787.8 90.6 0

PC2, TS, PC2 x TS 4 1787.8 90.7 0

a Number of parameter for each model includes the intercepts.

50

Table 3. Association between landscape structure (PC1-Forest integrity, PC2-Proportion of conifers), food treatment (FT;

supplementation or control), time elapsed since sunrise (TS; proportion (%) of total day length) and body mass index (pre-treatment

period) or log body mass index (treatment period) of black-capped chickadees. Data obtained at the center of the 24 landscapes during

pre-treatment (November) and treatment (January-March) periods for the two winters of the study. Model-averaged parameters (±

unconditional SE) were at first estimated from normal regressions. Parameter estimates for which zero is excluded from the 95%

unconditional confidence interval (effect size > 0) appear in bold.

Pre-treatment period Treatment period

PC1 0.0140 (0.0286) -0.0003 (0.0016)

PC2 0.0128 (0.0425) -0.0076 (0.0038)

TS 0.0040 (0.0010) 0.0004 (0.0001)

PC1 x TS 0.0001 (0.0006) 0.0000 (0.0000)

PC2 x TS -0.0008 (0.0007) 0.0000 (0.0001)

FT * -0.0172 (0.0073)

FT * x TS 0.0003 (0.0002)


51

Table 4. Association between landscape structure (PC1-Forest integrity, PC2-Proportion of conifers), food treatment (FT;

supplementation or control), time elapsed since sunrise (TS; proportion (%) of total day length) and fat score of black-capped

chickadees. Data obtained at the center of the 24 landscapes during pre-treatment (November) and treatment (January-March) periods

for the two winters of the study. Model-averaged parameters (± unconditional SE) were at first estimated from logistic regressions.

Parameter estimates for which zero is excluded from the 95% unconditional confidence interval (effect size > 0) appear in bold.

Pre-treatment period Treatment period

PC1 -0.0511 (0.3506) 0.0000 (0.0000)

PC2 -0.1650 (0.3690) 0.0000 (0.0000)

TS 0.0583 (0.0109) 0.0607 (0.0052)

PC1 x TS 0.0015 (0.0078) 0.0000 (0.0000)

PC2 x TS 0.0010 (0.0068) 0.0000 (0.0000)

FT * -0.5602 (0.2357)

FT * x TS 0.0037 (0.0048)


52

Patterns of daily fattening The relationship between time elapsed since sunrise and body mass index was linear

during the pre-treatment period (test for quadratic effect t160 = 0.71, P = 0.5) and during

the treatment period, in supplemented landscapes (test for quadratic effect t485 = -0.06, P

> 0.9) (Fig. 2). However, body mass index gain increased with daytime in control

landscapes (test for quadratic effect t63 = 2.32, P = 0.02) (Fig. 2).

The low sensitivity of fat score (Brown 1996) but moreover, the lack of information

concerning the exact relation between fat score and total body fat reserves in the black-

capped chickadee hinder the interpretation of exact patterns of daily fattening from fat

score data. Nevertheless, trends in plots illustrating the relation between time elapsed

since sunrise and fat score show similarities with patterns of fattening obtained from body

mass index data.

53

Pre-treatment period: all 24 landscapes

5.00

6.00

7.00

8.00

0 25 50 75 1000

1

2

3

4

5

0 25 50 75 100

Treatment period: 12 control landscapes

5.00

6.00

7.00

8.00

0 25 50 75 1000

1

2

3

4

5

0 25 50 75 100

Treatment period : 12 supplemented landscapes

5.00

6.00

7.00

8.00

0 25 50 75 1000

1

2

3

4

5

0 25 50 75 100

Bod

y m

ass i

ndex

(lef

t) or

fat s

core

(rig

ht)

Time elapsed since sunrise (% of total day length)

54

Figure 2. Relationship between time elapsed since sunrise (% of total day length) and

body mass index (BMI = 10 X Mass/Tarsus) or fat score of black-capped chickadees

during pre-treatment and treatment periods for the two winters of the study. A total of

161 birds were sampled during the pre-treatment period (November), while all 24

landscapes were still non-supplemented (R2 = 0.10). Respectively 64 and 486 birds were

sampled in the 12 control landscapes (R2 = 0.39) and in the 12 supplemented landscapes

(R2 = 0.16) during the treatment period (January-March). During the treatment period in

control landscapes, the inclusion of a quadratic term represented an improvement of 5.5

% in the explanation of body mass index variability by time elapsed since sunrise, and a

second-order polynomial curve was fitted as an indication of the relationship shape.

During the pre-treatment period and during the treatment period in supplemented

landscapes, the inclusion of a quadratic term did not improve the amount of variation

explained by more than 0.3 %.

55

Discussion

The landscape context Contrary to our first prediction, at no time during this study did we find landscape structure

to have an effect on chickadees’ condition. Maybe the use of independent observations of

birds of different ages and sexes, captured on different days in different environmental

conditions (e.g. weather) introduced too much noise to reveal any effect of landscape

structure. Landscape effects, if ever present at least in some part of the population under

study (e.g. subordinates), might have become apparent in a sample of marked individuals of

known dominance rank, captured several times over a daytime period, simultaneously in all

landscapes. Nevertheless, Telleria et al. (2001) also found no difference in the energetic

condition of blue tits Parus caerulescens when they compared during winter, populations

from small forest fragments and large contiguous forests in the milder climatic conditions

of central Spain. Therefore, maybe energetic costs associated to partially deforested

landscapes are indeed negligible. However, using ptilochronology (the measure of daily

feather growth) to evaluate the effect of woodlot area on the condition of four species of

woodland birds during winter in Ohio, Doherty & Grubb (2003) found a positive relation

between woodlot size and feather growth in Carolina chickadees Poecile carolinensis, but

not in the three other species they studied. Ptilochronology represents a powerful tool for

assessing the energetic condition of individuals in the wild over a period of several weeks

(e.g. Grubb 1989; Brodin & Ekman 1994). Contrary to daily fattening and other ineluctable

body maintenance constraints, the induced growth of one or two rectrices could withstand a

reduction in the energy allocated, without immediately compromising bird survival.

Landscape effects, if ever present here, perhaps would have become apparent with an

integrative approach spanning a much longer period of time such as ptilochronology.

The pre-treatment period In the relatively mild conditions of the pre-treatment period (mean minimum November

temperature, -3.5 °C; Environment Canada 2005), time elapsed since sunrise was the only

predictor variable affecting the energetic condition of chickadees. Body mass index gain

was rather linear from sunrise to sunset, in accordance with the results of Graedel &

56

Loveland (1995) but in contradiction with those of Lilliendahl (2002) both of whom also

studied hoarding species living in mild conditions. Our results are also contradictory to

model predictions for hoarding species (e.g. McNamara et al. 1990; Brodin 2000;

Pravosudov & Lucas 2001). However, these predictions primarily concern the mid-winter

period.

In November, the chickadees under study had to be moderately fat at dusk to survive the

fasting of the night (Fig. 2), in accordance with the prediction of Houston & McNamara

(1993) and other empiricists work (e.g. Macleod et al. 2005). This likely allowed them to

remain all day below a wing load threshold under which their ability to escape predators

was not compromised (Lind et al. 1999; Brodin 2000; Krams 2002), and to gain fat at a

rather constant rate, scattering the risk of predation throughout the daylight period, in

accordance with the “risk-spreading theorem” (Houston , McNamara & Hutchinson 1993).

Predation risk was real in our study area. The northern shrike Lanius excubitor (Linnaeus),

a major predator of black-capped chickadees during winter (reviewed by Smith 1991), was

indeed regularly observed over the course of this study.

The treatment period In the harsher conditions of the treatment period (mean minimum January-March

temperature, -13.1 °C; Environment Canada 2005), only food treatment and time elapsed

since sunrise had an effect (positive) on the energetic condition of birds. Using food to

attract birds to capture sites could induce a condition bias as individuals in poorest

condition could be most likely attracted and captured (Weatherhead & Greenwood 1981).

Dufour & Weatherhead (1991) suggested that during winter, food limitation might even

increase the likelihood of condition-bias. It is noteworthy that no such effect was apparent

here. A positive effect of food supplementation on the energetic condition of animals living

in cold environments has been reported in other studies concerning birds (e.g. Brittingham

& Temple 1988; Koivula et al. 2002; Rogers & Heath-Coss 2003), but also mammals (e.g.

Fauchald et al. 2004), and even fishes (e.g. Schultz & Conover 1999). However, theoretical

models, assuming that birds perceive a reduced starvation risk with an increase in resource

level, predict that body reserves should decrease, and hence associated costs of being fat,

57

when food is abundant or predictable (e.g. McNamara & Houston 1990; Houston &

McNamara 1993 but see Lima 1986). Fat reserves of birds living in warm environments

(low starvation risk) are indeed lower when food is abundant (see Rogers & Heath-Coss

2003).

Furthermore, not only did these birds take advantage early in the day of the overabundance

and predictability of resource provided by supplementation, but contrary to our prediction,

their body mass index gain was regular throughout the day. This pattern was also reported

by other empiricists who studied hoarding species of high latitudes having access to

supplemented food (e.g., Koivula et al. 2002) but again, contrasts with modellers’

predictions for food hoarders (e.g. McNamara et al. 1990; Brodin 2000; Pravosudov &

Lucas 2001). Why did birds in supplemented landscapes adopt such a strategy despite the

costs of being fat? In the black-capped chickadee, the safety margin provided by fat

reserves for surviving fasting in the cold is presumed to be less than a day (Chaplin 1974).

Therefore, we suggest that because of the periodical unforgiving cold spells and stormy

conditions (compromising access to the resource) prevailing in our study area, birds in

supplemented landscapes did not perceive such a reduced starvation risk but rather, that

they perceived starvation to represent a more proximate risk than predation. Their high

energetic condition throughout the daylight period would thus have represented a buffer

against stochastic periods of starvation due to weather unpredictability, in accordance with

the “unpredictable-episode hypothesis” (Pravosudov & Grubb 1997a). Furthermore, as

chickadees in supplemented landscapes likely remained close to the food source throughout

the day, and thus spent less time flying than those foraging in control landscapes, their costs

of being fat would have been reduced.

During the treatment period in control landscapes, chickadees delayed most of their

fattening toward the last half of the day. Such a strategy would appear risky, unless their

expected afternoon energy gain was predictable enough. This suggests that they then

retrieved food hoarded during the first part of the day. The general pattern we observed

offer support to the prediction of McNamara et al. (1990) for hoarding species. To our

knowledge, this represents the first empirical evidence supporting this model but

furthermore, reporting a seasonal change in the daily fattening pattern of a food-hoarding

58

species in natural conditions. Our results suggest that at this time of the year in the context

of the environmental conditions (severe climate, declining food supply) prevailing in our

study system, intensively foraging chickadees, by means of a reduced wing load,

minimized predation risk during the first half of the day but furthermore, mass-dependent

energy expenditure. As they did not benefit from an overabundance of food as in

supplemented landscapes (but see Pravosudov & Grubb 1998b), chickadees, by adopting

this energy-wise delayed fattening strategy, did not compromise their chances of being fat

enough at dusk to survive up to around 15 hours of fasting at night in our study area.

Chapitre III: Landscape-dependent response to predation risk by forest birds

Avertissement Le contenu de ce chapitre a été publié en mars 2003. Hormis quelques changements

mineurs dans le format ayant été nécessaires à la préparation de la thèse, le lecteur trouvera

ici toute l’information contenue dans :

Turcotte, Y, and A. Desrochers. 2003. Landscape-dependent response to predation risk by

forest birds. Oikos 100: 614-618.

60

Résumé Notre compréhension en profondeur des effets de la déforestation et de la fragmentation de

la forêt sur les effectifs des populations d’oiseaux nécessite la connaissance des

mécanismes sous-jacents. J’ai conçu une expérience dont le but était de déterminer si la

déforestation avait un effet sur le comportement anti-prédateur de Mésanges à tête noire

(Poecile atricapilla) en quête de nourriture. L’expérience a été effectuée à une bordure

forêt-champ, au centre de 24 paysages représentant globalement un gradient complet de

déforestation (8 – 88 % de couvert forestier, rayon de 500 m). J’ai mesuré la distance

maximale jusqu’à laquelle les bandes de mésanges s’aventuraient à l’intérieur du champ

pour aller chercher des graines de tournesol. J’ai utilisé cette valeur en guise d’indicateur de

leur propension à s’exposer au risque d’être victime d’un prédateur. Dans les paysages

témoins, les mésanges se sont aventurées plus loin à l’intérieur du champ lorsque le

déboisement était marqué, parfois même jusqu’à la distance maximale de 40 m imposée par

le dispositif expérimental. Dans les paysages expérimentaux, les mésanges avaient

bénéficié d’un approvisionnement en nourriture de plusieurs semaines avant la tenue de

l’expérience. Celles-ci, contrairement aux mésanges des paysages témoins, demeuraient

toujours près de la bordure, choisissant ainsi la protection conférée par le couvert forestier.

Ces résultats suggèrent que la déforestation augmente vraisemblablement les besoins

énergétiques des individus, ce qui aurait pour effet d’augmenter leur propension à s’exposer

à d’éventuels prédateurs.

61

Abstract

Knowing how forest loss and associated fragmentation actually impact individual birds is

essential to our understanding of consequences at the population level. We conducted a

landscape-level experiment to test whether deforestation affects the trade-off between

foraging and antipredatory behaviour of Black-capped Chickadees (Poecile atricapilla) in

24 landscapes (range 8 – 88 % forest cover, 500-m radius) during two winters. At a field-

forest edge in the centre of each landscape, we used the maximum distance ventured into

the open by flocks to get sunflower seeds placed on the snow-covered fields, as a measure

of risk-taking. In the more deforested landscapes, chickadees ventured farther (up to the

maximum of 40 m) into the open. Edge density and proportion of conifers in the forest had

no influence on risk-taking. However, where ad libitum food was available for a few weeks

prior to the experiment (in 12 of the 24 landscapes), chickadees ventured four meters or less

away from the forest edge, regardless of the level of deforestation. We conclude that

landscape deforestation increases energy stress, which in turn promotes risk-taking, and

may therefore increase winter mortality through greater exposure to predators.

62

Introduction Forest loss and associated fragmentation are probably the most publicised causes of the

decline of temperate, terrestrial bird populations (Terborgh 1989), despite the controversial

nature of current evidence (Haila et al. 1993, Harrison and Bruna 1999). Contributing to the

controversy is a limited understanding of how changes in landscapes actually impact

individuals. During the breeding season, deforestation and associated landscape changes

can reduce food supply (Burke and Nol 1998, Zanette et al. 2000), lead to lower nesting

success (Robinson et al. 1995, Crooks and Soulé 1999) and limit nesting opportunities

through edge avoidance during nest-site selection (Hunta et al. 1999). Deforestation may

further reduce mobility (Bélisle et al. 2001, Rodriguez et al. 2001) and therefore, could

compromise dispersal, recruitment and pairing success (Gibbs and Faaborg 1990, Villard et

al.1993) in relatively isolated forest patches. Other possible consequences of deforestation

on bird populations have received comparatively little attention, especially outside the

breeding season.

During winter at northern latitudes, forest birds must forage all day in order to survive sub-

zero temperatures. For several months, they have to cope with a decreasing food supply,

short daylight period and long fasting at night. When foraging, small endothermic birds

appear to trade-off energy gains with safety against predators (Lima and Dill 1990, Lima

and Bednekoff 1999). Food limitation (Boutin 1990, Doherty and Grubb 2002) and

predation (Jansson et al. 1981) are indeed the main factors causing mortality among them in

winter. In deforested landscapes, increased energetic cost of movements through gaps

(Hinsley 2000) or an unfavourable thermal environment (Dolby and Grubb 1999) will add

to energetic stress and can result in a lower survivorship (Doherty and Grubb 2002). Such

an ecological context is expected to promote foraging efficiency at the expense of

antipredatory behaviour (Houston and McNamara 1993).

We conducted a behavioural experiment to test whether landscape-level deforestation

affects the trade-off between feeding and safety from predators in small forest birds during

winter, using the Black-capped Chickadee (Poecile atricapilla), as a model species. This

small (~11 g) permanent-resident species inhabits forests throughout the northern part of

63

North America, where it forms winter flocks of 2-12 individuals (Smith 1991). Flocking

likely favours its survival as it presumably increases foraging efficiency and reduces

predation risk (Pulliam 1973, Matthysen 1990). Two other adaptations, food hoarding

(Shettleworth et al. 1995, Brotons et al. 2001) and hypothermia (Chaplin 1974, 1976) on

cold nights are assumed to help this species maintain its energetic balance and therein,

likely contribute to maximise its fitness.

64

Methods This study was conducted in January-February 2000 and 2001 on the south shore of the St.

Lawrence River estuary, in Kamouraska County, Quebec, Canada (47°30’ N, 69°50 W).

The study area covered approximately 600 km2 of agricultural landscape. Balsam Fir (Abies

balsamea), Quaking Aspen (Populus tremuloides), White Spruce (Picea glauca) and Paper

Birch (Betula papyrifera) dominate forest vegetation. The study area is part of the

Temperate Cold ecoclimatic region (Ecoregion Working Group 1989). At the La Pocatière

climate station, located within the study area, daily mean temperatures (1971-2000) for

January and February, the coldest winter months, are respectively -11.7 °C and –10.3 °C

(Environment Canada 2002). At this latitude, day length at winter solstice is 8 h 28 min.

In the context of a larger study of Black-capped Chickadee winter ecology, we selected 24

circular, 500-m radius, and non-overlapping landscapes, centred on a sharp edge between a

field and a forest. We chose this radius in order to include the core of most home ranges

(10-20 ha; reviewed by Smith 1991) of Black-capped Chickadee winter flocks occurring at

the centre of the landscapes. Based on a LANDSAT-7 satellite image taken in August 1999

and analysed with Patch Analyst (Elkie et al. 1999), these landscapes provided a broad

gradient of forest cover, edge density and proportion of conifers in the forest (Table 1, Fig.

1).

Table 1. Description of the 12 food-supplemented and the 12 control landscapes used in

behavioural trials.

Food-supplemented landscapes Control landscapes

Landscape metric Minimum Mean Maximum Minimum Mean Maximum

Forest cover (%) 8 45 88 10 53 87

Edge density (m/ha) 65 261 796 70 241 597

Conifers (%) 12 26 39 3 21 66

65

1C 3C 10C

Figure 1. Three control landscapes (500-m radius) used in behavioural trials. Forests are in

black, open habitats (mostly agricultural fields) in white. Each pixel corresponds to a 25 m

X 25 m surface. 1C: forest cover = 86 %, edge density = 73 m/ha. 3C: forest cover = 48 %,

edge density = 379 m/ha. 10C: forest cover = 10 %, edge density = 597 m/ha.

We established 12 pairs of adjacent landscapes with similar forest characteristics. Prior to

running the experiment, we provided sunflower seeds ad libitum at the field-forest edge at

the centre of one landscape of each pair for at least two weeks (hereafter, food-

supplemented landscapes). Chickadees respond quickly to supplemental feeding and their

local abundance becomes much higher than otherwise in such circumstances (Wilson

2001). The centre of the other landscape of each pair was located at least one kilometre

from the nearest habitation in order to reduce the likelihood of food supplementation from

another source (hereafter, control landscapes). Food treatments (food supplementation or

control) were conducted each winter.

Following the food supplementation period, we laid boards of wood (7 X 20 cm) every two

meters from the forest edge to 40 m into the snow-covered field at the centre of each 24

landscapes. On each board, we placed two sunflower seeds. The black seeds against the

pale board and the board itself against the snow offered strong visual contrast. In food-

supplemented landscapes, we removed the feeders just before behavioural trials were made

and covered with snow the seeds that fell on the ground. Chickadees were attracted to the

vicinity of the edge by remotely controlled playbacks of their mobbing calls (Turcotte and

Desrochers 2002). Playbacks ended and trials began when birds came within two meters of

the board nearest to the forest edge.

66

We assumed that vulnerability to predation increases with distance from the forest edge and

that birds were able to estimate predation risk. Therefore, we used the maximum distance

ventured into the open to pick up seeds by any flock member as a measure of risk-taking

(Caraco et al. 1980). Birds flying into the open without landing were not taken into

account; we considered that they did not expose themselves to predation as much as those

that landed on the boards (Hilton et al. 1999). Open habitats were assumed risky based on

observations, outside of trials, of two diurnal predators, most often the Northern Shrike

(Lanius excubitor) and once, the Northern Goshawk (Accipiter gentilis), in 33 % of both

food-supplemented and control landscapes. Predator sightings occurred in landscapes

where forest cover ranged from 8 to 87 % and were equally distributed between landscapes

where forest cover was above 50 % and those where it was below 50 %. The Northern

Shrike, which was observed in seven out of the 24 landscapes, could be considered as a

major predator of Black-capped Chickadees during winter (reviewed by Smith 1991).

Indeed, we found on a few occasions evidence of successful attacks by this predator within

and nearby the study area. Typically, birds depleted seeds progressively toward the open.

Trials ended when no bird landed on boards for 15 min. Consequently, the duration of trials

depended on flocks’ willingness to venture into the open: trials in which birds ventured

farther away into the open lasted longer than those in which birds took only the seeds

closest to the forest edge. We never conducted trials during the 2 hours following sunrise or

preceding sunset. Thus, we avoided a potentially confounding effect that could have arisen

otherwise if birds would have adopted a bimodal foraging routine, with feeding peaks near

dawn and dusk (McNamara et al. 1994). Trials were conducted once per landscape to avoid

behavioural habituation and to preserve the independence of the observations. Trials took

place on days without strong wind or precipitation.

We used generalised linear models (type III contrasts) to analyse the effects of landscape

metrics and food treatment on distance ventured into the open. The effects of potentially

confounding variables (flock size, air temperature, time of day; Caraco 1979, Grubb and

Greenwald 1982, McNamara et al. 1994) were addressed with Spearman rank correlations.

Our small sample sizes did not allow the incorporation of these variables and their

interactions with food treatment in more complex models. All statistical analyses were

made with SAS (SAS Institute Inc. 1999).

67

Results Neither the proportion of conifers in the forest nor edge density had an influence on

distance ventured into the open by chickadees. Thus, we eliminated those variables from a

preliminary model (Table 2). In preliminary and reduced (final) models, forest cover and

food treatment both affected distance ventured into the open but forest cover had different

effects in control and food-supplemented landscapes, as indicated by the interaction

between forest cover and food treatment (Table 2, Fig. 2). In fact, birds in the food-

supplemented landscapes invariably remained close (four meters or less) to the forest edge,

even though they were generally present in large numbers (at least 10 or more individuals).

Only birds in control landscapes ventured farther as forest cover decreased, some of them

even reaching the maximal distance allowed by our experimental design (40 m away from

the forest edge), where forest cover represented less than 50% of the landscape. The strong

effect of forest cover in control landscapes could not be explained by flock size (range = 1

to 8 birds, rs = -0.11, P = 0.7), air temperature (range = -17 to 0°C, rs = 0.00, P = 1.0), or

time of day (range = 9:15 to 14:00 EST, rs = 0.29, P = 0.4).

0

10

20

30

40

0 25 50 75 100

Forest cover within 500 m (%)

Dis

tanc

e ve

ntur

ed in

to th

e op

en (m

)

Figure 2. Relation between forest cover and maximum distance ventured into the open by

foraging Black-capped Chickadees in control (open circles) and food-supplemented (filled

circles) landscapes.

68

Table 2. Influence of food treatment and landscape structure on the maximum distance ventured into the open by foraging Black-

capped Chickadees in winter. Parameters estimated from generalised linear models with type III contrasts (n = 24).

Preliminary model Final model

Parameter Df Estimate (SE) F P Estimate (SE) F P

Food treatment1 1 36.96 (7.74) 22.78 <0.001 37.94 (7.66) 24.56 <0.0001

Forest cover (%) 1 -0.17 (0.16) 6.33 0.02 -0.03 (0.11) 7.67 0.01

Food treatment1 X Forest cover 1 -0.32 (0.15) 4.71 0.04 -0.35 (0.14) 5.82 0.03

Edge density (m/ha) 1 -0.02 (0.02) 1.43 0.25

Conifers (%) 1 -0.06 (0.11) 0.26 0.61

1 Food-supplementation used as reference category for parameter estimates.

69

Discussion Irrespective of forest cover, chickadees in food-supplemented landscapes remained close to

the forest and thus avoided predation risk at the expense of an immediate energy gain. As

they likely enjoyed higher energy reserves (in the form of fat or hoarded food) or because

they became conditioned to a less stochastic food supply, they could adopt a ‘safety-first’

strategy (Walther and Gosler 2001). By contrast, those in control landscapes ventured

farther into the open as forest cover decreased. This suggests that these birds faced

increased energy stress in deforested landscapes, thus leading them to forage in riskier

locations.

Though we did not evaluate variability of responses among flock members, other studies

suggest that socially subordinate individuals would be those who ventured first into the

open (De Laet 1985) or went farther from the forest edge (Schneider 1984) and thus, were

more exposed to predation. The population-level response to deforestation by wintering

chickadees may thus depend on whether subordinates would have breeding opportunities if

they survived (Desrochers et al. 1988). Nevertheless, considering flock members jointly,

safety against potential predators was compromised for greater foraging efficiency in the

more deforested landscapes. We suggest that this shift in the food versus safety trade-off

will amplify winter mortality of species living in cold environments.

Conclusion générale Cette étude a pu démontrer que la structure du paysage a des effets mesurables sur certains

aspects de l’écologie des oiseaux forestiers pendant l’hiver (répartition spatiale et

comportement anti-prédateur). Les résultats obtenus suggèrent de plus que les besoins

énergétiques élevés des oiseaux en cette saison sont en partie responsables des effets

observés puisque ceux-ci ont invariablement été différents lorsqu’une source

supplémentaire de nourriture était disponible.

Dans le premier chapitre de la thèse, les résultats présentés concernant la répartition spatiale

des oiseaux forestiers à la fin de l’automne sont en accord avec la vaste majorité des études

ayant rapportée une association positive entre la quantité d’habitat boisé et l’abondance des

individus et la richesse spécifique. Cependant pour la période correspondant au cœur de

l’hiver, la relation inverse observée dans les paysages témoins suggère fortement que

l’émigration des oiseaux a été facilitée dans les paysages les plus perméables, soit ceux

dont la couverture forestière était la plus élevée et la moins morcelée. Les résultats

divergents observés pendant le cœur de l’hiver dans les paysages expérimentaux, soit ceux

ayant bénéficié d’un apport supplémentaire en énergie alimentaire, suggèrent de plus que le

gain énergétique net pouvant être réalisé au cours de la quête alimentaire dans les paysages

témoins était vraisemblablement insuffisant et donc, à l’origine de la désertion partielle des

paysages témoins les plus boisés.

La condition énergétique et les patrons journaliers d’engraissement des Mésanges à tête

noire à la fin de l’automne et au cours de l’hiver ont fait l’objet du deuxième chapitre. Il

convient tout d’abord de mentionner que la structure du paysage n’a, en aucun cas, eu

d’effets mesurables sur la condition énergétique des mésanges, c'est-à-dire ni à la fin de

l’automne, ni au cours de l’hiver, que ce soit dans les paysages témoins ou dans les

paysages expérimentaux. Hormis le temps écoulé depuis le lever du soleil, seul l’apport

supplémentaire de nourriture a eu un effet mesurable sur la condition énergétique des

individus. Cet effet positif suggère que les oiseaux ont tiré profit de cette source de

nourriture et ce tout au long de la journée, probablement parce qu’en raison du caractère

stochastique des conditions météorologiques, le risque perçu de mort par inanition

71

demeurait plus immédiat que le risque d’être victime de prédateurs, tel que le suggère le

postulat du « unpredictable-episode hypothesis » (Pravosudov et Grubb 1997a). Ce résultat

supporte de plus l’hypothèse évoquée au paragraphe précédent selon laquelle, la nourriture

était en quantité insuffisante dans les paysages témoins et pouvait constituer un facteur

limitant les populations. Il est également possible que les déplacements rendus moins

importants pour les mésanges présentes dans les sites expérimentaux aient pu favoriser

l’adoption d’une telle stratégie d’engraissement, les coûts associés à une augmentation de la

masse (coût métabolique et risque de prédation accrus) se trouvant alors réduits.

Dans le contexte des conditions encore relativement clémentes de la fin de l’automne, avant

le début de l’approvisionnement en nourriture dans les paysages expérimentaux, le patron

journalier d’engraissement plutôt linéaire observé suggère que les mésanges ont alors

réparti tout au long de la journée les coûts associés à une masse plus élevée, en accord avec

le postulat du « risk-spreading theorem » de Houston et al. (1993). Plus tard dans l’hiver,

les mésanges des paysages témoins se devaient d’accumuler quotidiennement une plus

grande quantité de réserves graisseuses avant la nuit et ce, dans un contexte de rareté

croissante de la nourriture. Celle-ci ont alors accumulé ces réserves endogènes selon un

patron différent, la majeure partie de celles-ci étant rapidement accumulée pendant la

deuxième moitié de la journée, probablement en partie par la récupération des réserves

exogènes. Un patron semblable pour les espèces se constituant des caches de nourriture

avait été prédit par McNamara et al. (1990). Les résultats présentés ici représenteraient, à

ma connaissance, la première évidence empirique supportant ce modèle.

Le troisième chapitre présente les résultats d’une expérience qui s’est déroulé au cœur de

l’hiver et au cours de laquelle les Mésanges à tête noire ont été confrontées au choix d’aller

chercher de la nourriture jusqu’à 40 m à l’intérieur d’une ouverture, au détriment de la

sécurité conféré par le couvert forestier. Étant donné que les oiseaux devaient

obligatoirement s’immobiliser pour prendre possession de la nourriture, plus un oiseau

faisait le choix de s’aventurer loin de la lisière boisée, plus grande était la probabilité qu’il

soit capturé en cas d’attaque de la part d’un prédateur aérien déjà sur sa lancée. Les oiseaux

des paysages expérimentaux étaient temporairement privés de la nourriture fournie lors de

la réalisation de cette expérience. Néanmoins, ceux-ci sont toujours demeurés à proximité

72

de la forêt, vraisemblablement parce que leurs réserves énergétiques, tant corporelles que

sous la forme de caches, ne les incitaient pas à s’exposer au risque d’être victime d’un

prédateur, le risque perçu de mourir d’inanition étant alors trop faible. Par contre dans les

paysages témoins, les mésanges des paysages les plus sévèrement déboisés se sont

aventurées plus loin de la forêt que celles des paysages où la surface boisée était plus

importante, comme si elles percevaient que le risque de mourir par inanition était plus élevé

que celui d’être victime de prédation. Si les individus des milieux plus fragmentés sont

davantage enclins à s’exposer ainsi aux attaques des prédateurs, on peut supposer qu’une

mortalité plus élevée en découle (Hinsley et al. 1995).

Quelle vision d’ensemble peut-on avoir au terme de cette étude? De quelle manière les

résultats et interprétations présentés dans cette thèse s’articulent-ils les uns aux autres? Les

résultats des deux premiers chapitres nous suggèrent tout d’abord que, du point de vue des

oiseaux, les conditions environnementales caractérisant la fin de l’automne sont

suffisamment différentes de celles du cœur de l’hiver pour que, à moins d’un apport

énergétique supplémentaire sous la forme de nourriture, les patrons de répartition spatiale et

de l’engraissement journalier soient tout à fait différents entre ces deux périodes. Ces

résultats démontrent à quel point les décisions comportementales (émigration et stratégie

d’engraissement) des oiseaux forestiers pendant l’hiver sont largement tributaires du

rendement énergétique qu’ils peuvent obtenir lors de la quête alimentaire dans un habitat.

Les résultats de l’expérience présentée au troisième chapitre soulignent eux aussi

l’importance de la qualité de l’habitat d’un point de vue énergétique. Cependant, comment

interpréter le fait que, dans les paysages témoins, les mésanges des milieux où la

déforestation était la plus marquée et ayant pris davantage de risques n’étaient pas, à la

lumière des résultats présentés au chapitre II, en moins bonne condition énergétique? La

condition énergétique des mésanges a été évaluée dans cette étude à l’aide de deux

méthodes permettant d’estimer leurs réserves lipidiques totales à un moment précis de la

journée. Les résultats auraient-il été différents si la condition énergétique des oiseaux avait

été évaluée d’une manière moins ponctuelle soit en utilisant une approche intégrative de la

condition des oiseaux sur plusieurs semaines comme le permet la méthode

ptilochronologique (Grubb 1989, Brodin et Ekman 1994)? Il s’agirait certainement là d’une

avenue qu’il serait pertinent de considérer avant de conclure définitivement, surtout à la

73

lumière des résultats du troisième chapitre, que la structure du paysage n’a pas d’effets sur

la condition énergétique des oiseaux pendant l’hiver.

Applications écologiques Quelles sont les nouvelles connaissances issues de cette thèse pouvant contribuer aux

efforts de gestion des habitats forestiers et de conservation des populations d’oiseaux qui

les utilisent?

Les résultats présentés au premier chapitre suggèrent que les mouvements des populations

sont importants pendant l’hiver mais aussi, que les habitats forestiers fragmentés nuisent

alors à ces mouvements. Dès lors, un déboisement trop sévère pourrait avoir des

conséquences néfastes pour les populations dont les effectifs sont au départ limités, à moins

que ne soient préservés de longs corridors boisés ininterrompus, semblables à ceux qui sont

laissés en place le long des cours d’eau par l’industrie forestière dans les forêts publiques.

L’importance des bandes riveraines pour les mouvements de populations pendant l’été a

déjà été démontrée (Machtans et al. 1996) et leur importance présumée pour les

mouvements hivernaux a également été récemment évoquée par une équipe de chercheurs

de l’Ohio (Doherty et Grubb 2002). Étant donnée la richesse du réseau hydrographique

québécois, le potentiel des bandes riveraines à cet égard est particulièrement élevé, d’autant

plus qu’elles ne peuvent en aucun cas faire l’objet d’une coupe totale dans la forêt publique.

Cette dernière caractéristique leur confère un avantage indéniable sur le plan de la

conservation par rapport aux lisières boisées laissées en place entre les aires de coupe dans

la forêt publique. Ces dernières peuvent en effet être coupées dès que la régénération des

aires de coupe adjacentes atteint une hauteur de trois mètres, tel que le permet l’article 75

de la loi sur les forêts du gouvernement du Québec (Ressources naturelles et Faune 2005).

Il va sans dire que cette hauteur de trois mètres ne tient pas compte de la quantité de neige

pouvant couvrir le sol pendant l’hiver. Plus grandes seront ces accumulations, moins la

hauteur effective et donc la valeur réelle de ces peuplements en régénération, en tant

qu’habitats ou corridors pour l’avifaune, sera importante.

Cependant, les bandes riveraines se devraient d’être d’une largeur suffisamment importante

afin que ne soient pas compromises, par temps froid et venteux, la quête alimentaire (Grubb

74

1977) et donc ultimement, la survie des oiseaux qui les emprunteraient. À la lumière des

résultats d’études récentes (Dolby et Grubb 1999, Brotons et al. 1999), cette distance serait

vraisemblablement beaucoup plus grande que la largeur minimale de 20 mètres prévue

selon les dispositions de l’article 2 de la loi sur les forêts (Ressources naturelles et Faune

2005). Les effets protecteurs conférés par les bandes riveraines longeant les deux rives des

rivières les plus étroites peuvent certainement s’additionner, pour atténuer davantage les

effets du vent. Cependant, les oiseaux présents dans les bandes riveraines des lacs et des

rivières larges de plusieurs dizaines de mètres ne pourraient bénéficier de cet avantage. Si

en raison d’impératifs d’ordre économique, la largeur des bandes riveraines protégée par la

loi ne pouvait être augmentée de manière importante, des élargissements ponctuels mais

substantiels représenteraient, à tout le moins, une amélioration par rapport à la situation

actuelle.

Du troisième chapitre se dégage le constat qu’une détérioration de l’habitat peut modifier le

comportement anti-prédateur des individus. Dans les habitats forestiers fragmentés où des

oiseaux se rendront plus vulnérables aux attaques des prédateurs (Hinsley et al. 1995), que

ce soit en traversant les ouvertures ou en exploitant les ressources alimentaires des milieux

ouverts, les impacts négatifs sur les populations seront encore une fois potentiellement plus

marqués chez les espèces peu abondantes. La préservation par l’industrie forestière de

grandes surfaces boisées intactes s’avère dès lors essentielle à la conservation des espèces

résidentes les plus sensibles à la récolte forestière, telles que dans la forêt boréale, le Pic

tridactyle (Picoides tridactylus), le Pic à dos noir (Picoides arcticus) ou le Grimpereau brun

(Certhia americana) (Imbeau et al. 1999). Ces aires protégées se devraient alors de

correspondre au moins à la surface estimée des domaines vitaux hivernaux de ces espèces

plus vulnérables. Malheureusement, cette information de première importance ne nous est

pas toujours connue (Imbeau et Desrochers 2002).

Perspectives de recherche Les résultats présentés dans la présente thèse ne représentent qu’une contribution somme

toute modeste à notre compréhension de l’écologie hivernale des oiseaux forestiers

confrontés au phénomène de la déforestation. Pour le bénéfice des étudiants gradués ou

75

chercheurs établis qui prendront le relais afin d’améliorer encore davantage notre

compréhension de cette problématique, voici quelques suggestions de questions découlant

des thèmes abordés dans la thèse. Elles se prêtent toutes à la formulation d’hypothèses

falsifiables, certaines logistiquement réalistes pour un étudiant ne disposant que de une à

trois années tout au plus pour réaliser ses travaux d’échantillonnage, d’autres non. Notez

que la plupart de ces questions font référence à des oiseaux à l’état sauvage et qui ne

seraient pas nourris artificiellement.

Chapitre I

1) Quels sont les facteurs proximaux (p.ex. réchauffement momentané des températures

après une vague de froid, diminution de la quantité de la nourriture animale en deçà d’un

certain seuil, abondance des juvéniles dans la population) qui déclenchent au cours de

l’hiver, l’émigration d’une partie des populations résidentes?

2) Qui sont surtout ces émigrants? Les juvéniles? Les oiseaux subordonnés?

3) De tels mouvements existe-t-il également chez une espèce telle que la Mésange à tête

brune (Poecile hudsonica), étroitement associée aux peuplements fortement dominés par

les résineux?

4) Dans la mesure où ces mouvements de populations seraient davantage orientés vers le

sud, telle que le suggère la littérature anecdotique sur le sujet, se pourrait-il que les résultats

obtenus ici ne soient qu’un artéfact découlant de la présence de l’estuaire?

5) De plus, obtiendrait-on les mêmes résultats dans un contexte expérimental où, sur

plusieurs dizaines de kilomètres au-delà du rayon de 500 m retenu, le couvert forestier

serait parfaitement, autant que faire se peut, contrôlé?

Chapitre II

1) Il serait peut-être possible d’analyser les effets de la structure du paysage en considérant

la condition énergétique des mêmes individus, idéalement de statut hiérarchique connu,

76

capturés à quelques reprises au cours d’une même journée. Une telle approche, dite

longitudinale, nous conduirait-elle aux mêmes conclusions?

2) L’évaluation des effets de la structure du paysage sur la condition des Mésanges à tête

noire par une approche ptilochronologique (Grubb 1989, Brodin et Ekman 1994) nous

conduirait-elle aux mêmes conclusions?

3) Les relations de dominance à l’intérieur des bandes auraient-elles un effet sur la

croissance des plumes ainsi induite?

4) La relation entre le fat score et les réserves lipidiques ou la masse des individus d’une

même taille (ce qui éviterait de sacrifier des individus) est-elle linéaire comme cela semble

être le cas chez d’autres espèces (Blem 1990, Kullberg et al. 2000)

5) De quelle manière varient précisément, mois par mois, la condition énergétique et le

patron d’engraissement des Mésanges à tête noire?

6) De quelle manière varient non seulement mois par mois, mais aussi à différentes

latitudes, la condition énergétique et le patron d’engraissement des individus des espèces

résidentes les plus communes?

7) Les enregistrements des cris de houspillage de la Mésange à tête noire attirent-ils avec la

même efficacité les individus d’âge, de sexe et de condition énergétique différentes?

Chapitre III

1) Qui sont les individus qui, à l’intérieur des bandes, s’aventurent le plus loin dans les

ouvertures? Les juvéniles? Les oiseaux subordonnés?

2) Étant donné que le risque de mort par inanition varie pendant l’hiver, l’effet observé du

couvert forestier sur la propension des mésanges à s’exposer au risque d’être victime de

prédation varie-t-il d’un mois à l’autre, mais aussi à différentes latitudes?

3) De quelle manière les résultats seraient-ils influencés par une manipulation, non

seulement de la quantité de nourriture disponible, mais aussi, de sa prédictibilité.

77

4) Les individus vivant dans les forêts les plus fragmentées sont-ils morphologiquement

différents (p.ex. longueur relative des ailerons par rapport aux autres mesures corporelles)

des individus présents dans les milieux plus intègres et donc, plus aptes (adaptation

découlant des contraintes imposées par l’habitat: chez les adultes du moins, leur mue ayant

eu lieu dans l’environnement immédiat) au vol en milieu ouvert (voir Swaddle et Witter

1998)?

Annexe A

1) Serait-il possible d’améliorer la technique d’inventaire proposée (p. ex. période de

diffusion plus longue de l’enregistrement, pause entre deux séquences rapprochées de

diffusion)?

2) De quelle manière le couvert forestier (p. ex. proportion de résineux, importance du

couvert dans un rayon de quelques centaines de mètres) affecte-t-il l’efficacité de la

diffusion des cris de houspillage et la réponse des oiseaux?

Bibliographie Aberg, J., J. E. Swenson, and H. Andrén. 2000. The dynamics of Hazel Grouse (Bonasa bonasia L.) occurrence in habitat fragments. Canadian Journal of Zoology 78: 352-358. Ahlborn, B., and R. W. Blake. 2001. Why birds cannot be smaller than bees. Canadian Journal of Zoology 79: 1724-1726. Andrén, H. 1994. Effects of habitat fragmentation on birds and mammals in landscapes with different proportions of suitable habitat: a review. Oikos 71: 355-366. Anon. 1999. SAS Release 8.1. SAS Institute Inc, Cary. Anon. 2004. Canadian Climate Information. URL: http://www.msc-smc.ec.gc.ca/climate/. Austen, M. J. W., C. M. Francis, D. M. Burke, and M. S. W. Bradstreet. 2001. Landscape context and fragmentation effects on forest birds in southern Ontario. Condor 103:701-714. Bélisle, M., A. Desrochers, and M.-J. Fortin. 2001. Influence of forest cover on the

movements of forest birds: a homing experiment. Ecology 82: 1893-1904. Bent, A. C. 1946. Life histories of North American jays, crows, and titmice. U. S.

National Museum Bulletin 191. Berger, M., and J. S. Hart. 1974. Physiology and energetics of flight. Avian Biology 4: 415-477. Blake, J. G. 1987. Species-area relationships of winter residents in isolated woodlots. Wilson Bulletin 99: 243-252. Blem, C. R. 1984. Ratios in avian physiology. Auk 101: 153-155. Blem, C. R. 1990. Avian energy storage. Current ornithology 7: 59-113. Block, W. M., and L. A. Brennan. 1993. The habitat concept in ornithology: theory and

applications. Current Ornithology 11: 35-91. Boulinier, T. et al. 2001. Forest fragmentation and bird community dynamics: inference at regional scales. Ecology 82: 1159-1169. Boutin, S. 1990. Food supplementation experiments with terrestrial vertebrates: patterns,

problems, and the future. Canadian Journal of Zoology 68: 203-220. Brand, L. A., and T. L. George. 2001. Response of passerine birds to forest edge in coast redwood forest fragments. Auk 118: 678-686. Brewer, D., A. Diamond, E. J. Woodsworth, B. T. Collins, and E. H. Dunn. 2000. Canadian atlas of bird banding. Vol. 1: Doves, cuckoos, and hummingbirds through passerines, 1921-1995. Environment Canada, Canadian Wildlife Service, Ottawa. Brittingham, M. C., and S. A. Temple. 1988. Impacts of supplemental feeding on survival rates of Black-capped Chickadees. Ecology 69: 581-589. Broggi, J., M. Orell, E. Hohtola, and J.-A. Nilsson. 2004. Metabolic response to temperature variation in the great tit: an interpopulation comparison. Journal of Animal Ecology 73: 967-972. Brodin, A. 2000. Why do hoarding birds gain fat in winter in the wrong way? Suggestions from a dynamic model. Behavioral Ecology 11: 27-39. Brodin, A, and C. W. Clark, 1997. Long-term hoarding in the Paridae: a dynamicmodel. Behavioral Ecology 8: 178-185.

79

Brodin, A, and J. Ekman. 1994. Benefits of food hoarding. Nature 372: 510. Brotons, L., A. Desrochers, and Y. Turcotte. 2001. Food hoarding behaviour of black

capped chickadees (Poecile atricapillus) in relation to forest edges. Oikos 95: 511-519.

Brown, M. E. 1996. Assessing body condition in birds. Current ornithology 13: 67-135. Browning, M. R. 1995. Do Downy Woodpeckers migrate? Journal of Field Ornithology 66: 12-21. Burke, D. M., and E. Nol. 1998. Influence of food abundance, nest-site habitat, and forest

fragmentation on breeding Ovenbirds. Auk 115: 96-104. Burnham, K. P., and D. R. Anderson. 2001. Kullback-Leibler information as a basis for

strong inference in ecological studies. Wildlife Research 28: 111-119. Burnham, K. P., and D. R. Anderson. 2002. Model selection and multimodel inference: a

practical information-theoretic approach. 2nd edn. Springer-Verlag, New York. Calder, W. A. 1984. Size, function, and life history. Harvard University Press, Cambridge. Caraco, T. 1979. Time budgeting and group size: a test of theory. Ecology 60: 618-627. Caraco, T., S. Martindale, and H. R. Pulliam. 1980. Avian time budgets and distance to

cover. Auk 97: 872-875. Carey, C., and W. R. Dawson. 1999. A search for environmental cues used by birds in survival of cold winters. Current Ornithology 15: 1-31. Carrascal, L. M., J. A. Diaz, D. L. Huertas, and I. Mozetich. 2001. Behavioral thermoregulation by Treecreepers: trade-off between saving energy and reducing crypsis. Ecology 82: 1642-1654. Chaplin, S. B. 1974. Daily energetics of the Black-capped Chickadee, Parus atricapillus,

in winter. Journal of Comparative Physiology B 89: 321-330. Chaplin, S. B. 1976. The physiology of hypothermia in the Black-capped Chickadee,

Parus atricapillus. Journal of Comparative Physiology B 112: 335-344. Clark, C. W, and J. Ekman. 1995. Dominant and subordinate fattening strategies: a dynamic game. Oikos 72: 205-212. Cooper, C. B., and J. R. Walters. 2002. Experimental evidence of disrupted dispersal

causing decline of an Australian passerine in fragmented habitat. Conservation Biology 16: 471-478.

Cooper, S. J. 1999. The thermal and energetic significance of cavity roosting in Mountain Chickadees and Juniper Titmice. Condor 101: 863-866. Cooper, S. J., and J. A. Gessaman. 2004. Thermoregulation and habitat preference in mountain chickadee and juniper titmice. Condor 106: 852-861. Cooper, S. J., and D. L. Swanson. 1994. Seasonal acclimatization of thermoregulation in

the Black-capped Chickadee. Condor 96: 638-646. Crooks, K. R., and M. E. Soulé. 1999. Mesopredator release and avifaunal extinctions in

a fragmented system. Nature 400: 563-566. De Laet, J. F. 1985. Dominance and anti-predator behaviour of Great Tits Parus major: a

field study. Ibis 127: 372-377. Desrochers, A., M. Bélisle, and J. Bourque. 2002. Do mobbing calls affect the perception of predation risk by forest birds? Animal Behaviour 64: 709-714. Desrochers, A., S. J. Hannon, and K. E. Nordin. 1988. Winter survival and territory

acquisition in a northern population of Black-capped Chickadees. Auk 105: 727-736.

80

Doherty, P. F., Jr., and T. C. Grubb, Jr. 2000. Habitat and landscape correlates of presence, density, and species richness of birds wintering in forest fragments in Ohio. Wilson Bulletin 112: 388-394.

Doherty, P. F., Jr., and T. C. Grubb, Jr. 2002. Survivorship of permanent-resident birds in a fragmented landscape. Ecology 83: 844-857.

Doherty, P. F., Jr., and T. C. Grubb, Jr. 2003. Relationship of nutritional condition of permanent-resident woodland birds with woodlot area, supplemental food, and snow cover. Auk 120: 331-336. Dolby, A. S., and T. C. Grubb, Jr. 1999. Effects of winter weather on horizontal and

vertical use of isolated forest fragments by bark-foraging birds. Condor 101: 408-412.

Downes, C. M., et B. T. Collins. 2003. Le relevé des oiseaux nicheurs du Canada, de 1967 à 2000. Service canadien de la faune. Cahier de biologie No. 219. Dufour, K. W., and P. J. Weatherhead. 1991. A test of the condition-bias hypothesis using Brown-headed Cowbirds trapped during the breeding season. Canadian Journal of Zoology 69: 2686-2692. Dunn, E. H. 2001. Mass change during migration stopover: a comparison of species group and sites. Journal of Field Ornithology 72: 419-432. Dunn, E. H. 2002. A cross-Canada comparison of mass change in birds during migration stopover. Wilson Bulletin 114: 368-379. Ecoregion Working Group. 1989. Ecoclimatic regions of Canada, first approximation. Ecol. Land Classif. Series 23. Environment Canada, Ottawa. Elkie, P., R. Rempel, and A. Carr. 1999. Patch Analyst user’s manual. TM-002. Ontario

Ministry of Natural Resources, Northwest Science & Technology, Thunder Bay. Environment Canada. 2002. Canadian Climate and Water Information. URL:

http://www.msc-smc.ec.gc.ca/climate/. Environment Canada. 2005. Canadian Climate Information. URL:

http://www.climat.meteo.ec.gc.ca/climate_normals/. Fahrig, L. 1997. Relative effects of habitat loss and fragmentation on population

extinction. Journal of Wildlife Management 61: 603-610. Fauchald, P., T. Tveraa, C. Henaug, and N. Yoccoz 2004. Adaptive regulation of body reserves in reindeer, Rangifer tarandus: a feeding experiment. Oikos 107: 583- 591. Flather, C. H., and M. Bevers. 2002. Patchy reaction-diffusion and population abundance:

the relative importance of habitat amount and arrangement. American Naturalist 159: 40-56.

Frankino, W. A., B. J. Zwaan, D. L. Stern, and P. M. Brakefield. 2005. Natural selection and developmental constraints in the evolution of allometries. Science 307: 718- 720. Freemark, K. E., and B. Collins. 1992. Landscape ecology of birds breeding in temperate

forest fragments. In: Ecology and conservation of Neotropical migrant landbirds (J. M. Hagan, III, and D. W. Johnston, eds.), pp. 443-454. Smithsonian Institution Press, Washington.

Freemark, K. E., and H. G. Merriam. 1986. Importance of area and habitat heterogeneity to bird assemblages in temperate forest fragments. Biological Conservation 36: 115-141.

81

Gibbs, J. P., and J. Faaborg. 1990. Estimating the viability of Ovenbird and Kentucky Warbler populations in forest fragments. Conservation Biology 4: 193-196.

Gobeil, J.-F., and M.-A. Villard. 2002. Permeability of three boreal forest landscape types to bird movements as determined from experimental translocations. Oikos 98: 447-458. Gorney, E., W. S. Clark, and Y. Yom-Tov. 1999. A test of the condition-bias hypothesis yields different results for two species of sparrowhawks (Accipiter). Wilson Bulletin 111: 181-187. Gosler, A. G. 1996. Environmental and social determinants of winter fat storage in the great tit Parus major. Journal of Animal Ecology 65: 1-17. Graber, J. W., and R. R. Graber. 1979. Severe winter weather and bird populations in southern Illinois. Wilson Bulletin 91: 88-103. Graedel, S. K., and R. E. Loveland. 1995. Seasonal and diurnal mass variation in Black- capped Chickadees and White-throated Sparrows. Wilson Bulletin 107: 723-727. Graham, C. H. 2001. Factors influencing movement patterns of Keel-billed Toucans in a fragmented tropical landscape in southern Mexico. Conservation Biology 15: 1789-1798. Greenwood, J. G. 2003. Measuring sexual size dimorphism in birds. Ibis 145 (on-line): E124-E126. Grubb, T. C., Jr. 1977. Weather-dependent foraging behavior of some birds wintering in a

deciduous woodland: horizontal adjustments. Condor 79: 271-274. Grubb, T. C., Jr. 1989. Ptilochronology: feather growth bars as indicators of nutritional status. Auk 106: 314-320. Grubb, T. C., Jr., and P. F. Doherty, Jr. 1999. On home-range gap-crossing. Auk

116: 618-628. Grubb, T. C., Jr., and L. Greenwald. 1982. Sparrows and a brushpile: foraging responses

to different combinations of predation risk and energy cost. Animal Behaviour 30: 637-640.

Haila, Y. 2002. A conceptual genealogy of fragmentation research: from island biogeography to landscape ecology. Ecological Applications 12: 321-334.

Haila, Y., D. A. Saunders, and R. J. Hobbs. 1993. What do we presently understand about ecosystem fragmentation? In: Reconstruction of fragmented ecosystems: global and regional perspectives (Saunders, D. A., R. J. Hobbs, and P. R. Ehrlich, eds.), pp. 45-55. Surrey Beatty and Sons, Chipping Norton.

Hamel, P. B., W. P. Smith, and J. W. Wahl. 1993. Wintering bird populations of fragmented forest habitat in the Central Basin, Tennessee. Biological Conservation 66: 107-115. Hames, R. S., K. V. Rosenberg, J. D. Lowe, and A. A. Dhondt. 2001. Site reoccupation in fragmented landscapes: testing predictions of metapopulation theory. Journal of Animal Ecology 70: 182-190. Hanski, I. 1994. Patch-occupancy dynamics in fragmented landscapes. Trends in Ecology and Evolution 9: 131-135. Harrison, S., and E. Bruna. 1999. Habitat fragmentation and large-scale conservation:

what do we know for sure? Ecography 22: 225-232. Heinrich, B. 2003. Overnighting of Golden-crowned Kinglets during winter. Wilson

Bulletin 115: 113-114.

82

Hilton, G. M., W. Cresswell, and G. D. Ruxton. 1999. Intraflock variation in the speed of escape-flight response on attack by an avian predator. Behavioral Ecology 10: 391-395.

Hinsley, S. A. 2000. The costs of multiple patch use by birds. Landscape Ecology 15: 765-775.

Hinsley, S. A., P. E. Bellamy, and D. Moss. 1995. Sparrowhawk Accipiter nisus predation and feeding site selection by tits. Ibis 137: 418-420.

Houston, A. I., and J. M. McNamara. 1993. A theoretical investigation of the fat reserves and mortality levels of small birds in winter. Ornis Scandinavica 24: 205-219. Houston, A. I., J. M. McNamara, and J. M. C. Hutchinson. 1993. General results

concerning the trade-off between gaining energy and avoiding predation. Philosophical Transactions of the Royal Society B 341: 375-397.

Huhta, E., J. Jokimäki, and P. Rahko. 1999. Breeding success of Pied Flycatchers in artificial forest edges: the effect of a subobtimally shaped foraging area. Auk 116: 528-535.

Hurd, C. R. 1996. Interspecific attraction to the mobbing calls of Black-capped Chickadees (Parus atricapillus). Behavioural Ecology and Sociobiology 38: 287-292. Hurlbert, S. H. 1984. Pseudoreplication and the design of ecological field experiments.

Ecological Monographs 54: 187-211. Imbeau, L., and A. Desrochers. 2002. Foraging ecology and use of drumming trees by Three-toad Woodpeckers. Journal of Wildlife Management 66: 222-231. Imbeau, L., J.-P. Savard, and R. Gagnon. 1999. Comparing bird assemblages in successional black spruce stands originating from fire and logging. Canadian Journal of Zoology 77: 1850-1860. Jansson, C., J. Ekman, and A. von Brössen. 1981. Winter mortality and food supply in

tits Parus spp. Oikos 37: 313-322. Kaiser, A. 1993. A new multi-category classification of subcutaneous fat deposits of songbirds. Journal of Field Ornithology 64: 246-255. Kallander, H., and H. G. Smith. 1990. Food storing in birds: an evolutionary perspective. Current Ornithology 7: 147-207. Karr, J. R., J. D. Nichols, M. K. Klimkiewicz, and J. D. Brawn. 1990. Survival rates of birds of tropical and temperate forests: will the dogma survive? American Naturalist 136: 277- 291. Koenig, W. D., and J. M. H. Knops. 2001. Seed-crop size and eruptions of North

American boreal seed-eating birds. Journal of Animal Ecology 70: 609-620. Koivula, K., M. Orell, and K. Lahti. 2002. Plastic daily fattening routines in willow tits. Journal of Animal Ecology 71: 816-823. Kolozsvary, M. B., and R. K. Swihart. 1999. Habitat fragmentation and the distribution of amphibians: patch and landscape correlates in farmland. Canadian Journal of Zoology 77: 1288-1299. Krams, I. 2002. Mass-dependent take-off ability in wintering great tits (Parus major): comparison of top-ranked adult males and subordinate juvenile females. Behavioral Ecology and Sociobiology 51: 345-349. Kullberg, C., S. Jakobsson, and T. Fransson. 2000. High migratory fuel loads impair predator evasion in sedge warblers. Auk 117: 1034-1038. Lahti, K., M. Orell, S. Rytkönen, and K. Koivula. 1998. Time and food dependence in willow tit winter survival. Ecology 79: 2904-2916.

83

Lee, M., L. Fahrig, K. Freemark, and D. J. Currie. 2002. Importance of patch scale vs landscape scale on selected forest birds. Oikos 96: 110-118. Lehikoinen, E. 1987. Seasonality of the daily weight cycle in wintering passerines and its consequences. Ornis Scandinavica 18: 216-226. Liknes, E. T., S. M. Scott, and D. L. Swanson. 2002. Seasonal acclimatization in the

America Goldfinch revisited: to what extent do metabolic rates vary seasonally? Condor 104: 548-557. Lilliendahl, K. (2002) Daily patterns of body mass gain in four species of small wintering birds. Journal of Avian Biology, 33, 212-218. Lima, S. L. 1985. Maximizing feeding efficiency and minimizing time exposed to predators: a trade-off in the black-capped chickadee. Oecologia 66: 60-67. Lima, S. L. 1986. Predation risk and unpredictable feeding conditions: determinants of body mass in birds. Ecology 67: 377-385. Lima, S. L., and P. A. Bednekoff. 1999. Temporal variation in danger drives antipredator

behavior: the predation risk allocation hypothesis. American Naturalist 153: 649-659.

Lima, S. L., and L. M. Dill. 1990. Behavioral decisions made under the risk of predation: a review and prospectus. Canadian Journal of Zoology 68: 619-640.

Lind, J., T. Fransson, S. Jakobsson, and C. Kullberg. 1999. Reduced take-off ability in robins (Erithacus rubecula) due to migratory fuel load. Behavioral Ecology and Sociobiology 46: 65-70. Loery, G. and Nichols, J. D. 1985. Dynamics of a Black-capped Chickadee population,

1958-1983. Ecology 66: 1195-1203. Loery, G., K. H. Pollock, J. D. Nichols, and J. E. Hines. 1987. Age-specificity of Black- capped Chickadee survival rates: analysis of capture-recapture data. Ecology 68: 1038-1044. Machtans, C. S., M.-A. Villard, and S. J. Hannon. 1996. Use of riparian buffer strips as movement corridors by forest birds. Conservation Biology 10: 1366-1379. Macleod, R., P. Barnett, J. A. Clark, and W. Cresswell. 2005. Body mass change strategies in blackbirds Turdus merula: the starvation-predation risk trade-off. Journal of Animal Ecology 74: 292-302. Matthysen, E. 1990. Nonbreeding social organization in Parus. Current Ornithology

7: 209-249. Matthysen, E. 1993. Nonbreeding social organization in migratory and resident birds.

Current Ornithology 11: 93-141. Matthysen, E. 1999. Nuthatches (Sitta europaea: Aves) in forest fragments: demography

of a patchy population. Oecologia 119: 501-509. McKechnie, A. E., and B. G. Lovegrove. 2002. Avian facultative hypothermic responses:

a review. Condor 104: 705-724. McNamara, J. M., and A. I. Houston. 1980. The application of statistical decision theory

to animal behaviour. Journal of Theoretical Biology 85: 673-690. McNamara, J. M., and A. I. Houston. 1990. The value of fat reserves and the trade-off between starvation and predation. Acta Biotheorica 38: 37-61. McNamara, J. M., J. Ekman, and A. I. Houston. 2004. The effect of thermoregulatory

substitution on optimal energy reserves of small birds in winter. Oikos 105: 192-196.

84

McNamara, J. M., A. I. Houston, and J. R. Krebs. 1990. Why hoard? The economics of food storing in tits, Parus spp. Behavioral Ecology 1: 12-23. McNamara, J. M., A. I. Houston, and S. L. Lima. 1994. Foraging routines of small birds

in winter: a theoretical investigation. – Journal of Avian Biology 25: 287-302. Merom, K., Y. Yom-Tov, and R. McClery. 2000. Philopatry to stopover site and body condition of transient reed warblers during autumn migration through Israël. Condor 102: 441-444. Metcalfe, N. B., and S. E. Ure. 1995. Diurnal variation in flight performance and hence potential predation risk in small birds. Proceedings of the Royal Society of London B 261: 395-400. Middleton, A. L. A. 1986. Seasonal changes in plumage structure and body composition of the American Goldfinch, Carduelis tristis. Canadian Field-Naturalist 100: 545- 549. Pennycuick, C. J. 1989. Bird flight performance: a practical calculation manual. Oxford University Press, Oxford. Perkins, A. L., W. R. Clark, T. Z. Riley, and P. A. Vohs. 1997. Effects of landscape and weather on winter survival of Ring-necked Pheasant hens. Journal of Wildlife Management 61: 634-644. Pimm, S. L. 1998. The forest fragment classic. Nature 393: 23-24 Pravosudov, V. V., and T. C. Grubb, Jr. 1997a. Energy management in passerine birds during the nonbreeding season. Current ornithology 14: 189-234. Pravosudov, V. V., and T. C. Grubb, Jr. 1997b. Management of fat reserves and food caches in tufted titmice (Parus bicolor) in relation to unpredictable food supply. Behavioral Ecology 8: 332-339. Pravosudov, V. V., and T. C. Grubb, Jr. 1998a. Management of fat reserves in tufted titmice Baelophus bicolor in relation to risk of predation. Animal Behaviour 56: 49- 54. Pravosudov, V. V., and T. C. Grubb, Jr. 1998b. Management of fat reserves in tufted titmice (Parus bicolor): evidence against a trade-off with food hoards. Behavioral Ecology and Sociobiology 42: 57-62. Pravosudov, V. V., and J. R. Lucas. 2001. A dynamic model of short-term energy management in small food-caching and non-caching birds. Behavioral Ecology 12: 207-218. Pulliam, H. R. 1973. On the advantages of flocking. Journal of Theoretical Biology 38: 418-422. Pyle, P. 1997. Identification guide to North American birds. Part 1. Columbidae to

Ploceidae. Slate Creek Press, Bolinas. Redfern, C. P. F., A. E .J. Slough, B. Dean, J. L. Brice, and P. H. Jones 2000. Fat and body condition in migrating redwings Turdus iliacus. Journal of Avian Biology 31: 197-205. Ressources naturelles et Faune. 2005. Règlement sur les normes d’intervention dans les forêts du domaine de l’État. Loi sur les forêts. Éditeur officiel du Québec, Québec. Robbins, C. S., J. R. Sauer, R. S. Greenberg, and S. Droege. 1989. Population declines in North American birds that migrate to the neotropics. Proceedings of the National Academy of Sciences 86: 7658-7662.

85

Robinson, S. K., F. R. Thompson, III, T. M. Donovan, D. R. Whitehead, and J. Faaborg. 1995. Regional forest fragmentation and the nesting success of migratory birds. Science 267: 1987-1990. Rodewald, A. D., and R. H. Yahner. 2001. Influence of landscape composition on avian community structure and associated mechanisms. Ecology 82: 3493-3504. Rodriguez, A., H. Andrén, and G. Jansson. 2001. Habitat-mediated predation risk and

decision making of small birds at forest edges. Oikos 95: 383-396. Rogers, C. M. 2003. New and continuing issues with using visible fat classes to estimate fat stores of birds. Journal of Avian Biology 34: 129-133. Rogers, C. M., and R. Heath-Coss. 2003. Effect of experimentally altered food abundance on fat reserves of wintering birds. Journal of Animal Ecology 72: 822-830. Rogers, C. M., and J. N. M. Smith. 1993. Life-history theory in the non-breeding period: trade-offs in avian fat reserves. Ecology 74: 419-426. SAS Institute Inc. 1999. SAS Release 8.1. SAS Institute Inc, Cary. Schmidt, K. A. 2003. Nest predation and population declines in Illinois songbirds: a case for mesopredator effects. Conservation Biology 17: 1141-1150. Schmiegelow, F. K. A., and M. Mönkkönen. 2002. Habitat loss and fragmentation in dynamic landscapes: avian perspectives from the boreal forest. Ecological Applications 12: 375- 389. Schneider, K. J. 1984. Dominance, predation, and optimal foraging in White-throated

Sparrow flocks. Ecology 65: 1820-1827. Schultz, E. T., and D .O. Conover. 1999. The allometry of energy reserve depletion: test of a mechanism for size-dependent winter mortality. Oecologia 119: 474-483. Shettleworth, S. J., R. R. Hampton, and R. P. Westwood. 1995. Effects of season and

photoperiod on food storing by black-capped chickadees, Parus atricapillus. Animal Behaviour 49: 989-998.

Smith, S. M. 1984. Flock switching in chickadees: why be a winter floater? American Naturalist 123: 81-98.

Smith, S. M. 1991. The Black-capped Chickadee: behavioral ecology and natural history. Cornell University Press, Ithaca.

Sokal, R. R., and F. J. Rohlf. 1981. Biometry. 2nd edn. Freeman, San Francisco. Stephens, P. A., S. W. Buskirk, G. D. Hayward, and C. Martinez Del Rio. 2005. Information theory and hypothesis testing: a call for pluralism. Journal of Applied Ecology 42. 4-12. Stokes, M. E., C. S. Davis, and G. G. Koch. 2000. Categorical data analysis using the

SAS system. 2nd edn. SAS Institute Inc., Cary. Swaddle, J. P. and Witter, M. S. 1998. Cluttered habitats reduce wing asymmetry and increase flight performance in European starlings. Behavioral Ecology and Sociobiology 42: 281-287. Swanson, D. L. 1990. Seasonal variation of cold hardiness and peak rates of cold-induced thermogenesis in the Dark-eyed Junco (Junco hyemalis). Auk 107: 561-566. Taylor, P. D., L. Fahrig, K. Henein, and G. Merriam. 1993. Connectivity is a vital element of landscape structure. Oikos 68: 571-573. Telleria, J. L. and Santos, T. 1995. Effects of forest fragmentation on a guild of

wintering passerines: the role of habitat selection. Biological Conservation 71: 61- 67.

86

Telleria, J. L., E. Virgos, R. Carbonell, J. Pérez-Tris, and T. Santos 2001. Behavioural responses to changing landscapes: flock structure and anti-predator strategies of tits wintering in fragmented forests. Oikos 95: 253-264. Terborgh, J. W. 1989. Where have all the birds gone? Princeton University Press, Princeton. Thompson, F. R. III, and E. K. Fritzell. 1988. Ruffed Grouse winter roost site preference and influence on energy demands. Journal of Wildlife Management 52: 454-460. Turcotte, Y., and A. Desrochers. 2002. Playbacks of mobbing calls of Black-capped

Chickadees help estimate the abundance of forest birds in winter. Journal of Field Ornithology 73: 303-307.

Turcotte, Y., and A. Desrochers. 2003. Landscape-dependent response to predation risk by forest birds. Oikos 100: 614-618.

Turcotte, Y., and A. Desrochers. 2005. Landscape-dependent distribution of northern forest birds in winter. Ecography 28: 129-140. Van Wilgenburg, D. F. Mazerolle, and K. A. Hobson. 2001. Patterns of arthropod abundance, vegetation, and microclimate at boreal forest edge and interior in two landscapes: implications for forest birds. Écoscience 8: 454-461. Villard, M.-A., P. R. Martin, and C. G. Drummond. 1993. Habitat fragmentation and the

pairing success in the Ovenbird (Seirus aurocapillus). Auk 110: 759-768. Villard, M.-A., M. K. Trzcinski, and G. Merriam. 1999. Fragmentation effects on forest birds: relative influence of woodland cover and configuration on landscape occupancy. Conservation Biology 13: 774-783. Wallace, G. J. 1941. Winter studies of color-banded chickadees. Bird-Banding 12: 49-67. Walsberg, G. E. 1986. Thermal consequences of roost-site selection: the relative importance of three modes of heat conservation. Auk 103: 1-7. Walther, B. A., and A. G. Gosler. 2001. The effects of food availability and distance to

protective cover on the winter foraging behaviour of tits (Aves: Parus). Oecologia 129: 312-320.

Weise, C. M., and J. R. Meyer. 1979. Juvenile dispersal and development of site-fidelity in the Black-capped Chickadee. Auk 96: 40-55.

Weatherhead, P. J., and H. Greenwood. 1981. Age and condition bias of decoy-trapped birds. Journal of Field Ornithology 52: 10-15. Whitcomb, R. F., J. F. Lynch, M. K. Klimkiewicz, C. S. Robbins, B. L. Whitcomb, and D.

Bystrak. 1981. Effects of forest fragmentation on avifauna of the eastern deciduous forest. In: Forest island dynamics in man-dominated landscapes (R. L. Burgess, and D. M. Sharpe, eds.), pp. 125-205. Springer-Verlag, New York.

Wilson, W. H., Jr. 2001. The effects of supplemental feeding on wintering Black-capped Chickadees (Poecile atricapilla) in central Maine: population and individual responses. Wilson Bulletin 113: 65-72.

Winker, K. 1995. Autumn stopover on the isthmus of Tehuantepec by woodland Neartic-Neotropic migrants. Auk 112: 690-700.

Winker, K., D. W. Warner, and A. R. Weisbrod. 1992. Daily mass gains among woodland migrants at an island stopover site. Auk 109: 853-862.

Witter, M. S., and I. E. Cuthill. 1993. The ecological costs of avian fat storage. Philosophical Transactions of the Royal Society of London B 340: 73-90.

87

Yong, W., and F. R. Moore. 1993. Relation between migratory activity and energetic condition among thrushes (Turdinae) following passage across the Gulf of Mexico. Condor 95: 934-943.

Zanette, L., P. Doyle, and S. M. Trémont. 2000. Food shortage in small fragments: evidence from an area-sensitive passerine. Ecology 81: 1654-1666.

Zanette, L. 2001. Indicators of habitat quality and the reproductive output of a forest songbird in small and large fragments. Journal of Avian Biology 32: 38-46.

Annexe A: Playbacks of mobbing calls of Black- capped Chickadees help estimate the abundance of forest birds in winter

Avertissement Le contenu de la présente annexe a été publié à l’été de 2002. Hormis quelques

changements mineurs dans le format ayant été nécessaires à la préparation de la thèse, le

lecteur trouvera ici toute l’information contenue dans :

Turcotte, Y, and A. Desrochers. 2002. Playbacks of mobbing calls of Black-capped

Chickadees help estimate the abundance of forest birds in winter. Journal of Field

Ornithology 73: 303-307.

89

Résumé Beaucoup d’efforts ont été investis dans le développement de méthodes permettant

d’évaluer de manière précise les effectifs des populations d’oiseaux forestiers en période de

nidification. Cependant, beaucoup moins nombreuses ont été les études visant à

développer des méthodes applicables en dehors de cette période lorsque l’activité vocale

des oiseaux est minimale et que ceux-ci ne sont plus territoriaux. Nous avons évaluée une

nouvelle approche utilisable pour les inventaires des oiseaux forestiers devant être effectués

pendant l’hiver, soit l’utilisation d’un enregistrement de cris de houspillage de la Mésange

à tête noire (Poecile atricapilla). La diffusion de ce type d’enregistrement lors des points

d’écoute permettait la détection de plus d’individus, toutes espèces confondues et de plus

d’espèces, que lorsque ce type d’enregistrement n’était pas utilisé. En cette période de

l’année, l’heure du jour n’avait pas d’effet sur les résultats des inventaires, que

l’enregistrement soit diffusé ou non. En raison de leur efficacité, nous recommandons

l’utilisation de tels enregistrements lors des inventaires d’oiseaux forestiers devant être

conduits en dehors de la période de nidification.

90

Abstract Considerable attention has been devoted to the technical aspects of terrestrial bird surveys

during the breeding season. However, there is a paucity of information specifically

addressing the methodology of bird surveys at other times of the year when birds are less

vocal and are mobile over areas larger than territories. We tested a method for surveying

forest birds in winter, based on the use of playbacks of mobbing calls of Black-capped

Chickadees (Poecile atricapilla). When compared to pre-playback 5 min standard point

counts, playbacks of mobbing calls of the same duration allowed the detection of more

individuals and more species. Time of day, with or without playbacks, had no effect on the

number of individuals detected nor on species richness. We recommend the use of

playbacks of mobbing calls of Black-capped Chickadees for surveying forest birds during

the non-breeding season because of their efficiency.

91

Introduction Considerable attention has been devoted in the past to the methodological aspects of

terrestrial bird surveys during the breeding season (e.g., Ralph and Scott 1981, Ralph et al.

1995). The intense territorial singing activity of males then represents a key source of

information to estimate population numbers. However, much less information is available

concerning census procedures for the non-breeding season. During winter, birds are less

vocal and dwell over areas larger than territories. Therefore, a well-proven method such as

the fixed-radius point count conceived to survey birds during the breeding season becomes

inefficient (e.g., Fletcher et al. 2000). Other survey approaches such as unlimited distance

point counts (e.g., Gutzwiller 1991), transects (e.g., Rollfinke and Yahner 1990) or

mapping (e.g., Smith 1984) have been used during the non-breeding season but each of

these also presents some limitations (see Verner 1985). This situation may be in part

responsible for the relative paucity of bird community studies conducted in winter, despite

the critical importance of this time of year on bird survival (e.g., Desrochers et al. 1988,

Lahti et al. 1998).

In this paper we propose a new method for surveying resident forest bird communities in

winter. This method relies on the use of playbacks of mobbing calls of Black-capped

Chickadees (Poecile atricapilla). During winter, the Black-capped Chickadee is the most

abundant non-irruptive forest bird species in our study area as well as over much of the

northern part of North America. Its mobbing calls are given year round (Shedd 1983) and

are known to communicate the presence of predators to conspecifics as well as to other

species (mostly passerines; Hurd 1996) that whenever present, quickly aggregate around a

mobbing bird, often joining it in mobbing. For this reason, playbacks of mobbing calls of

Black-capped Chickadees have already been used with success in different situations (e.g.,

Desrochers and Hannon 1997, Gunn et al. 2000). The innovative aspect of the new census

method proposed is that the species targeted are not only those broadcast on playbacks as it

is normally the case (e.g., Falls 1981, Gibbs and Melvin 1993), but rather a large proportion

of those present in the community. To evaluate the value of this approach, we compared the

results obtained in counts during which we used playbacks of mobbing calls of Black-

capped Chickadees with those of fixed-radius standard point counts.

92

Study area and methods Surveys were conducted on the south shore of the St. Lawrence River estuary, in

Kamouraska County, Quebec, Canada (47° 30’ N, 69° 50’ W). The study area covered

approximately 600 km2 of agricultural landscape where forest vegetation is dominated by

Balsam Fir (Abies balsamea), Quaking Aspen (Populus tremuloides), White Spruce (Picea

glauca) and Paper Birch (Betula papyrifera). Based on a LANDSAT-7 satellite image

taken in August 1999 and analyzed with Patch Analyst (Elkie et al. 1999), forest cover

within 250 m of the center of the census sites ranges from 12 to 92%.

We conducted bird surveys at different times of day from sunrise to sunset, from November

to February during 3 consecutive winters (1998-1999 to 2000-2001) in 24 census sites.

These sites were selected in the context of a larger study of Black-capped Chickadee winter

ecology along a gradient of forest fragmentation. Census sites were separated from each

other by at least 2 km. The center of each census site was located at the intersection of a

forest, a field and a tertiary road. This type of roadside location is presumed to increase

detection rates, particularly of silent flying birds (Ralph et al. 1995). The observer (YT)

stood in the field about 10 m from the center of the census site while an assistant took note

of all birds seen or heard within a fixed-radius of 50 m. Birds flying above the forest were

not considered. Surveys were not conducted during heavy precipitations or when peak wind

speed, measured 1.5 m above ground with a hand-held anemometer, exceeded 30 km/h as

hearing ability then becomes limited and probability of detection decreases.

At each site, we used back-to-back two types of counts. We began with a 5 min standard

point count period (hereafter, pre-playback). It was immediately followed by another 5 min

point count period during which we playbacked mobbing calls of Black-capped Chickadees

(hereafter, playback). The mobbing chickadees were recorded with a parabola near Quebec

City when lured by a stuffed Screech Owl (Otus asio). Calls were playbacked with a 5 W

amplifier facing skyward, attached to a cassette player and placed on the ground at the

center of the census site. On days without wind, we were able to hear this recording from as

far away as about 200 m in the open. Sound level measured 1 m above the amplifier with a

sound level meter (RealisticTM) reached 105 decibels.

93

As each of the 24 census sites was surveyed 3 to 7 times within this period (no more than

one survey per month), data were averaged per site, in order to avoid pseudoreplication. To

test the hypothesis that playbacks resulted in higher counts, we used Wilcoxon Signed-

Ranks tests, appropriate for a paired comparisons design. Since the alternative hypothesis

was that playbacks increase the number of birds detected, tests were one-tailed. To evaluate

the effect of time of day on species richness and total number of individuals detected in the

surveys, we analyzed count results as a repeated-measures design with a Poisson

regression. We carried out statistical analysis using SAS (SAS Institute Inc. 1999).

94

Results

Comparison of pre-playback and playback counts Playbacks allowed us to detect more species, more individuals (all species combined) and

more Black-capped Chickadees (about 75% of all individuals detected) than did pre-

playback counts (Table 1). We also tested the effect of playbacks on species other than

Black-capped Chickadees that were present during the surveys. To reduce the risk of type II

statistical errors while performing analyses at the species level, we excluded species that

were present in less than 5 (20%) of the census sites (13 out of the 18 species recorded,

Table 2). Playbacks provided more observations of Golden-crowned Kinglet (Regulus

satrapa), Blue Jay (Cyanocitta cristata), and Downy Woodpecker (Picoides pubescens) but

not of Hairy Woodpecker (Picoides villosus) (Table 1).

Table 1. Mean (±SE) species richness and number of individuals of the most commonly

detected species during paired pre-playback and playback counts (5 min duration, 50 m

fixed-radius) within an agricultural landscape in Quebec during three consecutive winters,

1998-1999 to 2000-2001. Each census site (n = 24) was surveyed 3 to 7 times, but data

were averaged for each site before analysis. Type of count effects on species richness and

number of individuals were analyzed with one tailed Wilcoxon Signed-Ranks tests.

Type of count

Pre-playback Playback P

Species richness 0.29 ± 0.07 1.25 ± 0.11 <0.0001

Black-capped Chickadee 0.34 ± 0.10 3.63 ± 0.46 <0.0001

Golden-crowned Kinglet 0.07 ± 0.03 0.23 ± 0.07 0.006

Blue Jay 0 0.13 ± 0.06 0.004

Downy Woodpecker 0 0.05 ± 0.02 0.031

Hairy Woodpecker 0.01 ± 0.01 0.04 ± 0.02 0.218

Total number of individuals 0.45 ± 0.13 4.73 ± 0.57 <0.0001

95

Table 2. Number of census sites (out of the 24 of this study) where each species was

detected depending on the type of count.

Type of count

Species Pre-playback Playback Total

Black-capped Chickadee (Poecile atricapilla) 15 24 24

Golden-crowned Kinglet (Regulus satrapa) 5 11 13

Blue Jay (Cyanocitta cristata) 0 8 8

Downy Woodpecker (Picoides pubescens) 0 5 5

Hairy Woodpecker (Picoides villosus) 1 4 5

Boreal Chickadee (Poecile hudsonica) 0 4 4

Red-breasted Nuthatch (Sitta canadensis) 0 4 4

Pine Grosbeak (Pinicola enucleator) 0 2 2

Brown Creeper (Certhia americana) 0 2 2

Common Redpoll (Carduelis flammea) 0 2 2

American Goldfinch (Carduelis tristis) 0 1 1

Common Raven (Corvus corax) 0 1 1

Gray Jay (Perisoreus canadensis) 0 1 1

Northern Shrike (Lanius excubitor) 0 1 1

Pine Siskin (Carduelis pinus) 0 1 1

Three-toed Woodpecker (Picoides tridactylus) 1 1 1

Evening Grosbeak (Coccothraustes vespertinus) 1 0 1

Mourning Dove (Zenaida macroura) 1 0 1

96

Effect of time of day

There were no interactions between effects of count type and time of day whether species

richness (P = 0.42) or total number of individuals (P = 0.50) was considered as the

dependent variable. Therefore, irrespective of the type of count, time of day had no effect

on species richness nor on total number of individuals detected (Table 3).

Table 3. Mean (±SE) species richness and total number of individuals during paired pre-

playback and playback counts (5 min duration, 50 m fixed-radius) at different times of day

(EST) within an agricultural landscape in Quebec during three consecutive winters, 1998-

1999 to 2000-2001. Sample size refers to the number of census sites (out of the 24 of this

study) that were surveyed in each time of day category. There were no interactions between

effects of count type and time of day whether species richness or the total number of

individuals was considered as the dependent variable. Time of day effects on species

richness and total number of individuals results were analyzed with a repeated measures

design with a Poisson regression.

Species richnessa Total number of individualsb

Time of day n Pre-playback Playback Pre-playback Playback

Sunrise to 1000 h 18 0.23 ± 0.09 1.19 ± 0.19 0.26 ± 0.10 4.74 ± 1.07

1000-1200 h 20 0.28 ± 0.11 1.38 ± 0.20 0.50 ± 0.23 5.22 ± 1.05

1200-1400 h 20 0.31 ± 0.11 1.03 ± 0.17 0.56 ± 0.25 3.16 ± 0.86

1400 h to sunset 19 0.13 ± 0.07 0.88 ± 0.18 0.13 ± 0.07 3.13 ± 0.71

a P = 0.35. b P = 0.32.

97

Discussion The large amount of bird records that can be obtained efficiently by the use of playbacks of

mobbing calls of Black-capped Chickadees represents its main advantage. Indeed, not only

can it be used for the detection of individuals belonging to the mobbing species but also for

what seems to represent a large proportion (if not the majority) of the birds and species

present on a site (this study, Hurd 1996, Gunn et al. 2000). Playbacks could be particularly

useful for the detection of rarer or more secretive species all year round. As Gunn et al.

(2000) suggested, mobbing calls from another species (e.g., Carolina Chickadee (Poecile

carolinensis) or White-breasted Nuthatch (Sitta carolinensis)) would probably be as

effective in regions where Black-capped Chickadees are rare or absent.

Furthermore, results of playbacks (and of pre-playbacks as well) were not affected by time

of day. This suggests that the use of mobbing calls does not need to be restricted to the

early part of day as it is recommended for surveys conducted during the breeding season

(e.g., Ralph et al. 1993). However, pre-playback results are in contradiction with those

obtained in milder winter conditions by Rollfinke and Yahner (1990) who did fixed-width

transects in Pennsylvania (ca. 40°N) and by Gutzwiller (1991) who conducted unlimited-

distance point counts in Texas (ca. 31°N). Sampling throughout the day as we did, both of

their studies found that observed species richness was associated with time of day.

Rollfinke and Yahner (1990) also found that the total number of individuals was associated

with time of day (not tested by Gutzwiller 1991).

Despite the efficiency of the above playback method, its use by different researchers or

managers would probably yield less comparable results than those obtained by standard

point counts. Indeed, broadcasting equipment or recordings will obviously always present

differences of power or quality, associated with variation in the distance from which birds

are lured. Therefore, we recommend that playback counts should be preceded by standard

point counts in order to allow possible comparisons in time or space. Nevertheless, we

consider that the use of playbacks of mobbing calls for forest bird surveys conducted

outside the breeding season represents an efficient tool, particularly for species that are

uncommon or hard to detect.

98

Literature cited Desrochers, A., and S. J. Hannon. 1997. Gap crossing decisions by forest songbirds during the post-fledging period. Conservation Biology 11: 1204-1210. Desrochers, A., S. J. Hannon, and K. E. Nordin. 1988. Winter survival and territory acquisition in a northern population of Black-capped Chickadees. Auk 105: 727- 736. Elkie, P., R. Rempel, and A. Carr. 1999. Patch Analyst user’s manual. TM-002. Ontario Ministry of Natural Resources, Northwest Science & Technology, Thunder Bay. Falls, J. B. 1981. Mapping territories with playback: an accurate census method for songbirds. Studies in Avian Biology 6: 86-91. Fletcher, R. J., Jr, J. A. Dhundale, and T. F. Dean. 2000. Estimating non-breeding season bird abundance in prairies: a comparison of two survey techniques. Journal of Field Ornithology 71: 321-329. Gibbs, J. P., and S. M. Melvin 1993. Call-response surveys for monitoring breeding waterbirds. Journal of Wildlife Management 57: 27-34. Gunn, J. S., A. Desrochers, M.-A. Villard, J. Bourque, and J. Ibarzabal. 2000. Playbacks of mobbing calls of Black-capped Chickadees as a method to estimate reproductive activity of forest birds. Journal of Field Ornithology 71: 472-483. Gutzwiller, K. J. 1991. Estimating winter species richness with unlimited-distance point count. Auk 108: 853-862. Hurd, C. R. 1996. Interspecific attraction to the mobbing calls of Black-capped Chickadees (Parus atricapillus). Behavioural Ecology and Sociobiology 38: 287-292. Lahti, K., M. Orell, S. Rytkönen, and K. Koivula. 1998. Time and food dependence in Willow Tit winter survival. Ecology 79: 2904-2916. Ralph, C. J., and J. M. Scott, eds. 1981. Estimating numbers of terrestrial birds. Studies in Avian Biology No. 6. Ralph, C. J., S. Droege, and J. R. Sauer. 1995. Managing and monitoring birds using point counts: standards and applications. In: Monitoring bird populations by point counts. (C . J. Ralph, J. R. Sauer, and S. Droege, eds.), pp.161-168. General Technical Report PSW-GTR-149. Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture, Albany. Ralph, C. J., G. R. Geupel, P. Pyle, T. E. Martin, and D. F. DeSante. 1993. Handbook of field methods for monitoring landbirds. General Technical Report PSW-GTR-144. Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture, Albany. Rollfinke, B. F., and R. H. Yahner. 1990. Effects of time of day and season on winter bird counts. Condor 92: 215-219. SAS Institute Inc. 1999. SAS Release 8.1. SAS Institute Inc., Cary. Shedd, D. H. 1983. Seasonal variation in mobbing intensity in the Black-capped Chickadee. Wilson Bulletin 95: 343-348. Smith, P. G. R. 1984. Observer and annual variation in winter bird population studies. Wilson Bulletin 96: 561-574. Verner, J. 1985. Assessment of counting techniques. Current Ornithology 2: 247-302.

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